Guides
Intro to PHP Logging

How to Get Started with Logging in PHP

Better Stack Team
Updated on November 24, 2022

Logging is an essential and underutilized practice in software development. It's obvious value is for debugging, but it can also be a resource for deriving various analytics and usage information. When you learn how to log properly, you will be able to adequately track the inner workings of your application so that troubleshooting becomes much more effortless.

In this article, we will discuss the basics of logging in PHP and explore all the logging-related configurations you should know. We will begin by discussing the native logging functions in the language before branching out to examine the logging solutions developed by other PHP users. Here are some of the other things you stand to learn by following through with this article:

  • What is logging and examples of what to log.
  • Understanding PHP logging configurations.
  • Using native logging functions like error_log().
  • Why you should opt for a logging framework
  • An introduction to the Monolog framework for logging.

Prerequisites

Ensure to have the latest version of PHP installed on your machine before proceeding with this article. The code snippets and command output included in the sections below were all tested and confirmed to be accurate as at PHP v8.x, but they should continue to work with later versions.

What should you log?

Before we discuss how logging works in PHP, let's briefly examine what you should consider logging when developing your application. Some typical candidates for what to log include the following:

  • Errors and exceptions. Ensure to log every error and exception that occurs in your application so that you can find the root cause of an issue and fix them quickly.

  • Incoming requests. When a request is made to an endpoint in your application, you should log that event and include details such as a timestamp, user ID (if any), the endpoint and HTTP method, etc. It is also a good idea to generate a correlation ID at this point such that all other logging calls following from the request will include this ID, making it easier to trace the path of a specific client request in the application.

  • Any changes to your database, including inserting new data and updating and deleting existing data. You should record what data was changed, who changed it, and when it occurred.

  • Accessing sensitive information. Whenever sensitive or restricted information is being accessed on the system, a corresponding log entry should be recorded describing who accessed the resource and when.

Logging as much as possible does not mean you should record just anything as irrelevant entries will create noise and make your logs much less helpful. You should also take care never to log anything that would compromise user privacy such as passwords, credit card information, home addresses, phone numbers, or other Personally Identifiable Information (PII).

Understanding logging configurations in PHP

PHP saves all its configuration in a php.ini file whose location depends on your operating system and how you installed PHP. Luckily, there is an easy way to locate the configuration file. Open a new terminal window and run the following command:

php --ini
Copied!
Output
Configuration File (php.ini) Path: /opt/homebrew/etc/php/8.1
Loaded Configuration File: /opt/homebrew/etc/php/8.1/php.ini
Scan for additional .ini files in: /opt/homebrew/etc/php/8.1/conf.d Additional .ini files parsed: /opt/homebrew/etc/php/8.1/conf.d/ext-opcache.ini

The highlighted line above describes where to find the relevant php.ini file you need to edit. Copy the path to the file and open it in your text editor:

code <path/to/your/php.ini>
Copied!

This configuration file is enormous, but we only care about a small section. Scroll down to the "Error handling and logging" section or use your text editor's search function.

php.ini
. . .
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Error handling and logging ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
. . .
Copied!

Let's look at some of the common directives in this section that you need to note. You can find a complete list of all available directives in the official documentation. Note that you'll need to restart your web server after making changes to your php.ini file for the changes to take effect.

  • error_reporting: this directive configures the level of diagnostics that should be recorded, and its value should be an error level constant. For example, a value of E_ALL (the default) indicates that all diagnostic messages will be recorded regardless of their level, while a value of E_ALL & ~E_NOTICE means that all notice-level messages will be omitted.

  • display_errors: controls whether PHP should output errors to the screen as part of the program's output or if they should be hidden from view. Keeping this value On is fine in development settings, but it should always be turned Off in production, or end users may see ugly stack traces on your website when an error occurs (see above screenshot).

  • display_startup_errors: determines whether PHP will output errors that occur during its startup sequence. These errors are hidden by default, but you can turn them on when debugging.

  • log_errors: this option configures PHP to log errors to the location specified by the error_log directive. Setting this to Off will disable such behavior, but we recommend keeping this option On.

  • log_errors_max_len: restricts the maximum length of each log record to the specified value in bytes. The default is 1024 bytes, but setting this option to 0 removes the restriction.

  • error_log: this defines the path to a file where script errors should be logged. If this file does not exist, it will be automatically created. You can also forward log records to the system log by setting its value to syslog.

Aside from configuring these options directly in the php.ini file, PHP also offers a way override them at runtime. This can be useful in a serverless environment where you don't have access to the configuration file, or when you are troubleshooting an issue.

You can retrieve the current configuration using the ini_get() function as shown below:

<?php
echo 'log_errors = ' . ini_get('log_errors') . "\n";
?>
Copied!
Output
log_errors = 1

If you wish to override an existing configuration, you can provide a new value using the ini_set() function:

<?php
ini_set('log_errors', 1); // enable error logging
ini_set('error_log', 'app-errors.log' // specify error log file path
?>
Copied!

Error levels and constants in PHP

In this section, we'll examine one of the more obtuse aspects of PHP logging: error level constants. They are conceptually similar to log levels and are used to distinguish the different types of diagnostic messages produced when executing a program.

PHP has an unnecessarily complicated system when it comes to log levels. It uses many different constants to indicate different log levels, and these constants can be roughly classified into three families:

1. E_ for internal PHP errors

These include the following:

  • E_ERROR: a fatal error that causes script termination (e.g., calling a non-existent function, out of memory, etc.).
  • E_WARNING: a runtime warning that doesn't terminate script execution (e.g., using an undefined variable in an expression).
  • E_PARSE: compile-time parse errors, usually syntax errors.
  • E_NOTICE: runtime notices, meaning PHP encountered something that it thinks could be a mistake, but could also be intentional. For instance, using unassigned values in your code.

The constants above are used to indicate the severity of any event in your application, and they are not user-customizable. When something happens, PHP will automatically create a log record and decide if it is E_ERROR, E_WARNING, or E_NOTICE. The only thing we can do with these constants is to use them in the error_reporting directive to tell PHP whether or not to log these errors.

2. E_USER for application-specific logs

  • E_USER_ERROR: something went seriously wrong with your project, causing services to stop working.
  • E_USER_WARNING: something abnormal happened, but it doesn't affect the core functionalities of your applications, though the situation may need to be addressed soon to prevent it from escalating further.
  • E_USER_NOTICE: informative messages that describe the normal operation of the program.

3. LOG_ for system logs

The LOG_ family conforms to the standard syslog severity levels:

  • LOG_EMERG: the entire application is unusable.
  • LOG_ALERT: something serious happened that needs to be addressed immediately.
  • LOG_CRIT - a critical function is no longer working.
  • LOG_ERR - an error occurred, but the application can continue working.
  • LOG_WARNING: abnormal situations that may later become an error if not addressed.
  • LOG_NOTICE: unusual events but not error conditions.
  • LOG_INFO - informative messages.
  • LOG_DEBUG: used to indicate messages helpful for debugging.

Exploring logging functions in PHP

Now that we've examined the PHP logging configuration and its error constants, let's get into the nitty-gritty of logging in a PHP project. Several logging functions ship with PHP, and they all depend on the configurations discussed in the earlier sections.

To guarantee that you get results consistent with what is described in the sections below, ensure that your logging configuration matches the following values:

php.ini
. . .
error_reporting = E_ALL & ~E_DEPRECATED & ~E_STRICT
display_errors = On
log_errors = On
error_log = error.log
. . .
Copied!

Go to your working directory and create a new logging.php file using the command below. This is where we are going to explore PHP's logging functions throughout this tutorial.

code logging.php
Copied!

The most basic form of logging involves sending messages to the console. In PHP, we can easily print to the console using echo or print statements like this:

logging.php
<?php
echo "This is a log message.\n";

print "This is also a log message.";
?>
Copied!
Output
This is a log message.

This is also a log message.

While echo and print statements are valuable ways to print some text to the console, you shouldn't use them for logging as there are much better facilities for doing so. We will explore some of these functions in detail here.

The error_log() function

PHP's error_log() function pushes a log message to the file path defined in error_log config option. You can also specify a different file path directly in the function, if you prefer. It can take up to four arguments but only the first one is required:

error_log(
    string $message,
    int $message_type = 0,
    ?string $destination = null,
    ?string $additional_headers = null
): bool
Copied!
  1. The $message argument is the log message you wish to record.
  2. The $message_type argument has a confusing name but it specifies where you want PHP to push the message. It has four possible values:
    • 0: the message is sent to the location specified in the error_log directive.
    • 1: the message is pushed through email, which is specified by the $destination parameter.
    • 3: the message is appended to the file specified by the $destination parameter.
    • 4: the message is sent directly to the Server API (SAPI) logging handler, which depends on your platform. For example, Apache's error log will be used for LAMP (Linux Apache, MySQL, PHP) setups.
  3. The $destination parameter specifies where the log message should be sent, and it could be an email address or a local file, depending on the value of $message_type.
  4. The $additional_headers parameter is only used when $message_type is set to 1. It is the same as the $additional_headers used in PHP's mail() function.

Let's go ahead and use the error_log() function to create a log record like this:

logging.php
<?php
error_log("database not available!");
?>
Copied!

Once you execute this script, you will notice an error.log file in the current directory:

php logging.php
Copied!
ls
Copied!
Output
error.log logging.php

Examine the contents of the error.log file through the cat command as shown below:

cat error.log
Copied!
Output
[27-Jul-2022 16:05:49 America/New_York] database not available!

As you can see, the error_log() function automatically includes a timestamp which is one reason why using a dedicated logging function is better than echo and print statements.

You can also record a log entry into a different file by changing the $message_type and $destination parameters as shown below:

logging.php
. . .
error_log("database not available!", 3, "my-errors.log");
Copied!

Save the changes and execute the logging.php file again.

php logging.php
Copied!

This time, the log entry will be sent to the my-errors.log file:

cat my-errors.log
Copied!
my-errors.log
database not available!
Copied!

Notice that a timestamp is not included in the above entry! This is because the method bypasses the operating system's logging mechanism leading to less detailed logs. A better way to log to a different file is by using the ini_set() function discussed earlier:

logging.php
<?php
. . .
ini_set("error_log", "my-errors.log");
error_log("database not available!");
?>
Copied!

This example will produce the same result, but with the timestamp included:

my-errors.log
[27-Jul-2022 17:38:04 America/New_York] database not available!
Copied!

The trigger_error() function

The trigger_error() function is used to record application-specific errors, warnings and notices. It takes two parameters: the first one is the log message, and the second one defines its error level (one of E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, or E_USER_DEPRECATED):

logging.php
<?php
trigger_error("A user requested a resource.", E_USER_NOTICE);
trigger_error("The image failed to load!", E_USER_WARNING);
trigger_error("User requested a profile that doesn't exist!", E_USER_ERROR);
?>
Copied!

When you execute the script above, you will observe the following console output:

Output
Notice:  A user requested a resource. in /home/eric/test/logging.php on line 2
Warning:  The image failed to load! in /home/eric/test/logging.php on line 3
Fatal error:  User requested a profile that doesn't exist! in /home/eric/test/logging.php on line 4

The above logs are also recorded to the error.log file in a slightly different format:

cat error.log
Copied!
Output
[8-Jul-2022 22:11:43 America/New_York] PHP Notice:  A user requested a service. in /home/eric/test/logging.php on line 3
[8-Jul-2022 22:11:43 America/New_York] PHP Warning:  The image failed to load! in /home/eric/test/logging.php on line 4
[8-Jul-2022 22:11:43 America/New_York] PHP Fatal error:  User requested a profile that doesn't exist! in /home/eric/test/logging.php on line 5

Each log entry specifies the corresponding log level and the location of the log message. If you don't use a user-assigned error level constant (prefixed with E_USER_), an error will be thrown.

<?php
trigger_error("A user requested a service.", E_ERROR);
?>
Copied!
Output
Fatal error: Uncaught ValueError: trigger_error(): Argument #2 ($error_level) must be one of E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, or E_USER_DEPRECATED in /home/ayo/dev/betterstack/betterstack-community/demo/logging-php/logging.php:2
Stack trace:
#0 /home/ayo/dev/betterstack/betterstack-community/demo/logging-php/logging.php(2): trigger_error()
#1 {main}
  thrown in /home/ayo/dev/betterstack/betterstack-community/demo/logging-php/logging.php on line 2

Logging to the system log

A third way to log messages in PHP is using the syslog() function. It sends the log message to the default system log like this:

syslog(LOG_ERR, "Custom error message");
Copied!

The first argument to syslog() is the appropriate log level constant, and the second is the log message to be recorded. Notice that when specifying the log level, the third group of constants (those prefixed with LOG_) are utilized. These constants are unique to the syslog() function, and they are all listed in its documentation.

Here are a few ways to view your system log:

  • On Linux, use one of the following commands to access the system log (depending on the specific OS):
tail -f /var/log/syslog
Copied!
tail -f /var/log/messages
Copied!
journalctl -f
Copied!
  • On macOS, run tail -f /var/log/system.log in your terminal.
  • On Windows, use the Windows Event Viewer.

Here's some sample output from the system log on a Linux-based OS after executing the previous code snippet:

Output
Jul 27 20:58:28 fedora php[485047]: Custom error message

You can customize the output above through the openlog() function. It opens a connection to the system logger and takes up to three arguments: a string prefix to be added to each message, the logging options, and the logging facility. After calling openlog(), you should call closelog() to close the connection.

openlog("MyAppPrefix", LOG_PID | LOG_PERROR,LOG_USER);
syslog(LOG_ERR, "Custom error message");
closelog();
Copied!

Here's the output after executing the script above:

Output
Jul 28 09:20:04 fedora MyAppPrefix[596277]: Custom error message

You can learn more about system logs on Linux by reading our detailed tutorial on the subject.

Why you should use a logging framework

Using PHP's built-in logging functions is an easy way to get started with logging in simple projects, but they are insufficient for creating a robust logging strategy in serious applications due to their lack of features and flexibility, such as customizing the formatting of your logs, or determining how they are recorded. They also cannot capture a wide variety of data types since everything fed into them must be a string, which means you have to do extra work to capture contextual data in a log entry.

These limitations are why third-party logging frameworks are prevalent in the PHP ecosystem. Here are some of the most popular logging libraries for PHP that are worth exploring for production-grade applications:

  • Monolog: Monolog is the most popular PHP logging framework out there. It has dozens of handlers that can send your log records to files, emails, databases, Slack, etc. It is also packed with multiple formatters that allow you to customize your logs however you want.
  • Analog: Analog is a minimal logging library. It also has several handlers that can send your logs to different destinations. However, it does not have optional features such as formatters and processors.
  • KLogger: Unlike Analog, KLogger goes in a different direction. It can only push your logs to a file, but it has many different formatters that you can use to customize the timestamp, file name, file extension, context information, etc.

In the next section, we will briefly examine how to implement logging using the Monolog library as it's the most popular and satisfactorily meets all our criteria for a good logging framework.

Getting started with Monolog

To use Monolog in your PHP application, you need to install the library through PHP Composer:

composer require monolog/monolog
Copied!
Output
. . .
Package operations: 2 installs, 0 updates, 0 removals
  - Installing psr/log (3.0.0): Extracting archive
- Installing monolog/monolog (3.1.0): Extracting archive
11 package suggestions were added by new dependencies, use `composer suggest` to see details. Generating autoload files 1 package you are using is looking for funding. Use the `composer fund` command to find out more!

This command will install Monolog into the vendor directory, and you can import the package into your project like this:

logging.php
<?php

require __DIR__."/vendor/autoload.php"; // This tells PHP where to find the autoload file so that PHP can load the installed packages

use Monolog\Logger; // The Logger instance
use Monolog\Handler\StreamHandler; // The StreamHandler sends log messages to a file on your disk
use Monolog\Level; // Log levels
?>
Copied!

The three classes we imported into the file above represent Monolog's most important concepts. First, logging in Monolog is channel-based, and the Logger instance is used to initialize a new log channel which provides a mechanism for grouping different logs. For example, you can have an errors channel that records errors, a performance channel that logs performance-related messages, and so on. You can organize this system however you like depending on what kind of application you are building.

Each log channel can be assigned multiple handlers, and they are responsible for sending log messages to various destinations. For example, the StreamHandler above can push your messages to the console or a local file.

Finally, each handler needs to have a minimum log level, which defines the minimum level a message must have to be logged by the handler. Monolog provides a more standard way of dealing with log levels than PHP's error level constants. There are eight different levels available in Monolog, and they are modeled after the Syslog severity levels discussed earlier:

  • EMERGENCY
  • ALERT
  • CRITICAL
  • ERROR
  • WARNING
  • NOTICE
  • INFO
  • DEBUG

Logging in PHP with Monolog

Let's look at an example of how to log messages to the console with Monolog:

logging.php
<?php
. . .
// New Logger instance. Create a new channel called "my_logger".
$logger = new Logger("my_logger");

// Create a new handler. In this case, it is the StreamHandler, which will send the log messages to the console.
$stream_handler = new StreamHandler("php://stdout", Level::Debug);

// Push the handler to the log channel
$logger->pushHandler($stream_handler);

// Log the message
$logger->debug("This is a debug message.");
$logger->info("This is an info message.");
$logger->error("This is an error message.");
$logger->critical("This is a critical message.");
?>
Copied!

Execute the logging.php file, and observe the following output:

Output
[2022-07-11T03:32:57.111007+02:00] my_logger.DEBUG: This is a debug message. [] []
[2022-07-11T03:32:57.111750+02:00] my_logger.INFO: This is an info message. [] []
[2022-07-11T03:32:57.111941+02:00] my_logger.ERROR: This is an error message. [] []
[2022-07-11T03:32:57.112127+02:00] my_logger.CRITICAL: This is a critical message. [] []

In this example, a new log channel called my_logger was initialized using the Logger instance, and then a StreamHandler() instance was assigned to it. This handler was setup to log debug-level messages or higher to the standard output (represented by php://stdout).

Lastly, several log messages are recorded by using the debug(), info(), error(), and critical() methods which give the message the corresponding log level that is observed in the output. Notice that a timestamp is also included with each entry.

Instead of logging to the terminal console, we can also push the log record to a local file using the StreamHandler(). All we need to do is change the destination parameter like this:

logging.php
<?php
. . .

$logger = new Logger("my_logger");

$stream_handler = new StreamHandler(__DIR__ . "/log/debug.log", Level::Debug);
$logger->pushHandler($stream_handler); $logger->debug("This is a debug message."); ?>
Copied!

This causes log messages using the $logger channel to be sent to a /log/debug.log file. You can examine its contents with the following command:

cat ./log/debug.log
Copied!
Output
[2022-07-10T01:53:24.848775+02:00] my_logger.DEBUG: This is a debug message. [] []

There are many other handlers other than the StreamHandler demonstrated above, so ensure to read our detailed guide on Monolog to learn more about them and how to create a custom handler.

Formatting your logs

From the examples above, you will observe that all the log records follow a predefined format. They all start with a timestamp, followed by the log level, message, context and extra information. This format might not fit your needs when creating a logging system, so Monolog provides several formatters that you can use to customize the log records.

One of the useful formatters to look at is the LineFormatter. It takes a log record as input, and subsequently outputs a formatted string of the record. Let's take a look at an example:

logging.php
. . .
use Monolog\Level;
use Monolog\Logger;
use Monolog\Handler\StreamHandler;
use Monolog\Formatter\LineFormatter;
$logger = new Logger("my_logger"); $stream_handler = new StreamHandler("php://stdout", Level::Debug);
$output = "%level_name% | %datetime% > %message% | %context% %extra%\n";
$stream_handler->setFormatter(new LineFormatter($output));
$logger->pushHandler($stream_handler); $logger->debug("This file has been executed."); ?>
Copied!

In this case, we are making Monolog return a log record that starts with the log level, then the date, message, and context information.

Output
DEBUG | 2022-07-10T21:33:51.345896+02:00 > This file has been executed. | [] []

Besides rearranging the different segments, it is also possible for us to customize the timestamp output, since the default format isn't very human readable.

logging.php
<?php
. . .
$dateFormat = "Y-n-j, g:i a";
$output = "%level_name% | %datetime% > %message% | %context% %extra%\n"; $stream_handler->setFormatter(new LineFormatter($output, $dateFormat)); . . . ?>
Copied!
Output
DEBUG | 2022-7-10, 10:24 pm > This file has been executed. | [] []

In a production environment, your application will probably generate tons of logs, and in this case, it is best to use a [structured logging format][how-to-start-logging-with-monolog#using-a-structured-log-format-json] that is better suited to parsing by automated logging tools. Using Monolog's JsonFormatter, you are able to log in JSON format, making it easier for machines to read.

logging.php
<?php

require __DIR__ . "/vendor/autoload.php";

use Monolog\Level;
use Monolog\Logger;
use Monolog\Handler\StreamHandler;
use Monolog\Formatter\JsonFormatter;

$logger = new Logger("daily");

$stream_handler = new StreamHandler("php://stdout", Level::Debug);
$logger->pushHandler($stream_handler);
$stream_handler->setFormatter(new JsonFormatter());
$logger->debug("This is a debug message.");
Copied!
Output
{"message":"This is a debug message.","context":{},"level":100,"level_name":"DEBUG","channel":"daily","datetime":"2022-07-29T21:43:30.910327+02:00","extra":{}}

Notice that the log message, the log level, the timestamp, and so on have all been turned into JSON data.

You can learn more about formatters in Monolog and their available options by reading their documentation.

Final thoughts

We covered a lot of ground in this article, beginning with PHP's native logging functions and their quirks and features before briefly discussing logging frameworks and why you need one. We also introduced Monolog, a logging library for PHP application and demonstrated some of its basic features that help you create a more useful logging system. There's a lot more about Monolog that can't be covered here so we recommend diving into our detailed guide to logging with Monolog to get a full picture of what you can do with it.

Thanks for reading, and happy logging!

Centralize all your logs into one place.
Analyze, correlate and filter logs with SQL.
Create actionable
dashboards.
Share and comment with built-in collaboration.
Got an article suggestion? Let us know
Next article
How to Get Started with Monolog Logging in PHP
Monolog is among the most popular pieces of open source software, providing logging capabilities for PHP applications.
Licensed under CC-BY-NC-SA

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.