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Xdebug's built-in profiler allows you to find bottlenecks in your script and visualize those with an external tool such as KCacheGrind or WinCacheGrind.

Introduction #

Xdebug's Profiler is a powerful tool that gives you the ability to analyse your PHP code and determine bottlenecks or generally see which parts of your code are slow and could use a speed boost. The profiler also collects information about how much memory is being used, and which functions and methods increase memory usage.

The profiler in Xdebug outputs profiling information in the form of a Cachegrind compatible file. This allows you to use the excellent KCacheGrind tool (Linux, KDE) to analyse your profiling data. If you are on Linux you can install KCacheGrind with your favourite package manager.

If you are on Windows, there are precompiled QCacheGrind binaries available. (QCacheGrind is KCacheGrind without KDE bindings).

If you are on Mac OSX, homebrew has a formula for QCacheGrind.

There is also a web based front-end called Webgrind.

In case you can not use KDE (or do not want to use KDE) the kcachegrind package also comes with a perl script "ct_annotate" which produces ASCII output from the profiler trace files.

Starting The Profiler #

In your php.ini, 99-xdebug.ini, or other distribution specific PHP ini file, change (or set) the xdebug.mode setting to profile. Please refer to Configure PHP in the documentation to find out which PHP ini file to modify.

This instructs Xdebug to start writing profiling information into the directory configured with the xdebug.output_dir directive. The name of the generated file always starts with "cachegrind.out." and ends by default with the PID (process ID) of the PHP (or Apache) process. You can change the filename format with xdebug.profiler_output_name. Make sure you have enough space in your xdebug.output_dir as the amount of information generated by the profiler can be enormous for complex scripts.

You can also selectively enable the profiler by setting xdebug.start_with_request#trigger to trigger. You can then enable the profiler by using an environment value, a GET/POST parameter, or COOKIE variable of the name XDEBUG_TRIGGER. The Chrome, Firefox, and Safari extensions that can be used to enable the step debugger (see HTTP Debug Sessions) can also be used with this setting.

Xdebug adds the HTTP header X-Xdebug-Profile-Filename to a request that is being profiled. This header contains the name of the file that holds the profiling information for that request.

Analysing Profiles #

After a profile information file has been generated you can open it with KCacheGrind:

Once the file is opened you have plenty of information available in the different panes of KCacheGrind. On the left side you find the "Flat Profile" pane showing all functions in your script sorted by time spent in this function, and all its children. The second column "Self" shows the time spent in this function (without its children), the third column "Called" shows how often a specific function was called and the last column "Function" shows the name of the function. Xdebug changes internal PHP function names by prefixing the function name with "php::" and include files are handled in a special way too. Calls to include (and include_once, require and require_once) are followed by "::" and the filename of the included file. In the screenshot on the left you can see this for "include::/home/httpd/ez_34/v..." and an example of an internal PHP function is "php::mysql_query". The numbers in the first two columns can be either percentages of the full running time of the script (like in the example) or absolute time (1 unit is 1/1.000.000th of a second). You can switch between the two modes with the button you see on the right.

The pane on the right contains an upper and lower pane. The upper one shows information about which functions called the current selected function ("eztemplatedesignresource->executecompiledtemplate in the screenshot). The lower pane shows information about the functions that the current selected function called.

The "Cost" column in the upper pane shows the time spent in the current selected function while being called from the function in the list. The numbers in the Cost column added up will always be 100%. The "Cost" column in the lower pane shows the time spent while calling the function from the list. While adding the numbers in this list up, you will most likely never reach 100% as the selected function itself will also takes time to execute.

The "All Callers" and "All Calls" tabs show not only the direct calls from which the function was called respectively all directly made function calls but also function calls made more levels up and down. The upper pane in the screenshot on the left shows all functions calling the current selected one, both directly and indirectly with other functions inbetween them on the stack. The "Distance" column shows how many function calls are between the listed and the current selected one (-1). If there are different distances between two functions, it is shown as a range (for example "5-24"). The number in parentheses is the median distance. The lower pane is similar except that it shows information on functions called from the current selected one, again either direct or indirect.

Related Content #

Related Settings and Functions #

Settings #

string xdebug.log = #

Configures Xdebug's log file.

Xdebug will log to this file all file creations issues, Step Debugging connection attempts, failures, and debug communication.

Enable this functionality by setting the value to a absolute path. Make sure that the system user that PHP runs at (such as www-data if you are running with Apache) can create and write to the file.

The file is opened in append-mode, and will therefore not be overwritten by default. There is no concurrency protection available.

The log file will include any attempt that Xdebug makes to connect to an IDE:

[2693358] Log opened at 2020-09-02 07:19:09.616195
[2693358] [Step Debug] INFO: Connecting to configured address/port: localhost:9003.
[2693358] [Step Debug] ERR: Could not connect to debugging client. Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port).
[2693358] [Profiler] ERR: File '/foo/cachegrind.out.2693358' could not be opened.
[2693358] [Profiler] WARN: /foo: No such file or directory
[2693358] [Tracing] ERR: File '/foo/trace.1485761369' could not be opened.
[2693358] [Tracing] WARN: /foo: No such file or directory
[2693358] Log closed at 2020-09-02 07:19:09.617510

It includes the opening time (2020-09-02 07:19:09.616195), the IP/Hostname and port Xdebug is trying to connect to (localhost:9003), and whether it succeeded (Connected to client). The number in brackets ([2693358]) is the Process ID.

It includes:

process ID in brackets
2020-09-02 07:19:09.616195
opening time

For Step Debugging:

INFO: Connecting to configured address/port: localhost:9003.
ERR: Could not connect to debugging client. Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port).

For Profiling:

ERR: File '/foo/cachegrind.out.2693358' could not be opened.
WARN: /foo: No such file or directory

For Function Trace:

ERR: File '/foo/trace.1485761369' could not be opened.
WARN: /foo: No such file or directory

All warnings and errors are described on the Description of errors page, with detailed instructions on how to resolve the problem, if possible. All errors are always logged through PHP's internal logging mechanism (configured with error_log in php.ini). All warnings and errors also show up in the diagnostics log that you can view by calling xdebug_info().

Step Debugger Communication

The debugging log can also log the communication between Xdebug and an IDE. This communication is in XML, and starts with the <init XML element:

    xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug"
    language="PHP" xdebug:language_version="7.4.11-dev"
    protocol_version="1.0" appid="2693358" idekey="XDEBUG_ECLIPSE">
        <engine version="3.0.0-dev"><![CDATA[Xdebug]]></engine>
        <author><![CDATA[Derick Rethans]]></author>
        <copyright><![CDATA[Copyright (c) 2002-2020 by Derick Rethans]]></copyright>

The fileuri attribute lists the entry point of your application, which can be useful to compare to breakpoint_set commands to see if path mappings are set-up correctly.

Beyond the <init element, you will find the configuration of features:

<- feature_set -i 4 -n extended_properties -v 1
-> <response
       xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug"
       command="feature_set" transaction_id="4" feature="extended_properties" success="1">

And continuation commands:

<- step_into -i 9
-> <response
       xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug"
       command="step_into" transaction_id="9"
       status="break" reason="ok">
           <xdebug:message filename="file:///home/httpd/www.xdebug.org/html/router.php" lineno="3">

You can read about DBGP - A common debugger protocol specification at its dedicated documation page.

The xdebug.log_level setting controls how much information is logged.

Many Linux distributions now use systemd, which implements private tmp directories. This means that when PHP is run through a web server or as PHP-FPM, the /tmp directory is prefixed with something akin to:


This setting can additionally be configured through the XDEBUG_CONFIG environment variable.

integer xdebug.log_level = 7 #

Configures which logging messages should be added to the log file.

The log file is configured with the xdebug.log setting.

The following levels are supported:

0CriticalsErrors in the configuration
1ErrorsConnection errors
3WarningsConnection warnings
5CommunicationProtocol messages
7InformationInformation while connecting
10DebugBreakpoint resolving information

Criticals, errors, and warnings always show up in the diagnostics log that you can view by calling xdebug_info().

Criticals and errors are additionally logged through PHP's internal logging mechanism (configured with error_log in php.ini).

This setting can additionally be configured through the XDEBUG_CONFIG environment variable.

string xdebug.mode = develop #

This setting controls which Xdebug features are enabled.

This setting can only be set in php.ini or files like 99-xdebug.ini that are read when a PHP process starts (directly, or through php-fpm). You can not set this value in .htaccess and .user.ini files, which are read per-request, nor through php_admin_value as used in Apache VHOSTs and PHP-FPM pools.

The following values are accepted:

Nothing is enabled. Xdebug does no work besides checking whether functionality is enabled. Use this setting if you want close to 0 overhead.
Enables Development Helpers including the overloaded var_dump().
Enables Code Coverage Analysis to generate code coverage reports, mainly in combination with PHPUnit.
Enables Step Debugging. This can be used to step through your code while it is running, and analyse values of variables.
Enables Garbage Collection Statistics to collect statistics about PHP's Garbage Collection Mechanism.
Enables Profiling, with which you can analyse performance bottlenecks with tools like KCacheGrind.
Enables the Function Trace feature, which allows you record every function call, including arguments, variable assignment, and return value that is made during a request to a file.

You can enable multiple modes at the same time by comma separating their identifiers as value to xdebug.mode: xdebug.mode=develop,trace.

XDEBUG_MODE environment variable

You can also set Xdebug's mode by setting the XDEBUG_MODE environment variable on the command-line; this will take precedence over the xdebug.mode setting, but will not change the value of the xdebug.mode setting.

Some web servers have a configuration option to prevent environment variables from being propagated to PHP and Xdebug.

For example, PHP-FPM has a clear_env configuration setting that is on by default, which you will need to turn off if you want to use XDEBUG_MODE.

Make sure that your web server does not clean the environment, or specifically allows the XDEBUG_MODE environment variable to be passed on.

string xdebug.output_dir = /tmp #

The directory where Xdebug will write tracing, profiling, and garbage collection statistics to. This directory needs to be writable for the system user with which PHP is running.

This setting can be changed in php.ini, .htaccess (and equivalent files), and within a PHP file with ini_set().

In some cases (when profiling, or when xdebug.start_with_request=yes with tracing), Xdebug creates the file before the script runs. In that case, changes made through ini_set() will not be taken into account.

This setting can additionally be configured through the XDEBUG_CONFIG environment variable.

integer xdebug.profiler_append = 0 #

When this setting is set to 1, profiler files will not be overwritten when a new request would map to the same file (depending on the xdebug.profiler_output_name setting. Instead the file will be appended to with the new profile.

string xdebug.profiler_output_name = cachegrind.out.%p #

This setting determines the name of the file that is used to dump traces into. The setting specifies the format with format specifiers, very similar to sprintf() and strftime(). There are several format specifiers that can be used to format the file name.

See the xdebug.trace_output_name documentation for the supported specifiers.

This setting can additionally be configured through the XDEBUG_CONFIG environment variable.

string xdebug.start_with_request = default #

A Function Trace, Garbage Collection Statistics, Profiling, or Step Debugging can be activated at the start of a PHP request. Whether this happens depends on the value of this setting:


The functionality starts when the PHP request starts, and before any PHP code is run.

For example xdebug.mode=trace and xdebug.start_with_request=yes starts a Function Trace for the whole request.


The functionality does not get activated when the request starts.

You can still start a Function Trace with xdebug_start_trace(), Step Debugging with xdebug_break(), or Garbage Collection Statistics with xdebug_start_gcstats().

The Profiling will never activate with this value.


The functionality only gets activated when a specific trigger is present when the request starts.

The name of the trigger is XDEBUG_TRIGGER, and Xdebug checks for its presence in either $_ENV (environment variable), $_GET or $_POST variable, or $_COOKIE (HTTP cookie name).

There is a legacy fallback to a functionality specific trigger name: XDEBUG_PROFILE (for Profiling), XDEBUG_TRACE (for a Function Trace), and XDEBUG_SESSION (for Step Debugging).

There is another legacy trigger for Step Debugging only. If you set the XDEBUG_CONFIG environment variable to any value, then the step debugger will also get activated.

Debug session management for Step Debugging is also available through XDEBUG_SESSION_START.

With xdebug.trigger_value you can control which specific trigger value will activate the trigger. If xdebug.trigger_value is set to an empty string, any value will be accepted.


The default value depends on xdebug.mode:

  • debug: trigger

  • gcstats: no

  • profile: yes

  • trace: trigger

string xdebug.trigger_value = "" #

This setting can be used when xdebug.start_with_request is set to trigger, which is the default for Step Debugging and Function Trace.

In trigger mode, Xdebug will only start its functionality when the XDEBUG_TRIGGER is set in the environment, or when the XDEBUG_TRIGGER GET, POST, or COOKIE variable is set.

The legacy names XDEBUG_SESSION (for Step Debugging), XDEBUG_PROFILE (for Profiling), and XDEBUG_TRACE (for Function Trace) can also be used instead of XDEBUG_TRIGGER.

Normally, Xdebug does not look at which value is actually used. If this setting is set to a non-empty string, then Xdebug will only trigger if the value matches the value of this setting.

With the following settings:


Xdebug's profiler will only start when either the environment variable XDEBUG_TRIGGER is set to StartProfileForMe, the GET or POST variable XDEBUG_TRIGGER is set to StartProfileForMe, or when the cookie XDEBUG_TRIGGER has the value StartProfileForMe.

From Xdebug 3.1, it is possible to configure multiple values by using a comma separated list. In that case, Xdebug will trigger if the supplied value matches any of the entries that are configured through this setting:


See also:

For how the triggering mechanism works, and which environment and server variables Xdebug acts on.

boolean xdebug.use_compression = true #

Introduced in Xdebug >= 3.1

If enabled, the Function Trace and Profiling features will create GZip compressed files as output. This reduces diskspace.

If GZip compression is not supported by Xdebug, because it was not compiled in, then Xdebug will add a warning to its log and xdebug_info() diagnostics section.

It is enabled by default if Xdebug has GZip support, and disable if Xdebug does not have GZip support.

The QCacheGrind tool that you can use to visualise profiling information does not support reading GZip compressed profile files, whereas KCacheGrind and PhpStorm do. If you are a QCacheGrind user, you should set xdebug.use_compression to false.

Functions #

xdebug_get_profiler_filename() : mixed #

Returns the profile information filename

Returns the name of the file which is used to save profile information to, or false if the profiler is not active.

xdebug_info( string $category = null ) : mixed #

Show and retrieve diagnostic information

This function presents APIs to retrieve information about Xdebug itself. Which information gets returned, or displayed, depends on which arguments, or none at all, are given.

$category =

Without arguments, this function returns an HTML page which shows diagnostic information. It is analogous to PHP's phpinfo() function.

The HTML output includes which mode is active, what the settings are, and diagnostic information in case there are problems with debugging connections, opening of files, etc.

Each warning and error in the diagnostics log also links through to the Description of errors documentation page.

$category = 'mode' (New in Xdebug 3.1)

The function returns an array of all the enabled modes, whether through xdebug.mode or the XDEBUG_MODE environment variable.


xdebug_info'mode' ) );


array(3) {
  [0] =>
  string(5) "debug"
  [1] =>
  string(7) "develop"
  [2] =>
  string(5) "trace"

$category = 'extension-flags' (New in Xdebug 3.1)

The function returns an array of all the compile flags that were enabled when running ./configure as part of Xdebug's compilation process.

The only flag that is available, is the compression flag. If this flag is enabled, then the xdebug.use_compression setting is available, and enabled by default.

Profiling and Function Trace will create GZip compressed files if the xdebug.use_compression setting is turned on (the default).


xdebug_info'extension-flags' ) );


array(1) {
  [0] =>
  string(11) "compression"