If xdebug.client_discovery_header is configured to be a non-empty string, then the value is used as key in the $_SERVER superglobal array to determine which header to use to find the IP address or hostname to use for 'connecting back to'. This setting is only used in combination with xdebug.discover_client_host and is otherwise ignored.

For example, if xdebug.client_discovery_header is set to HTTP_FORWARD_HOST, then Xdebug will check $_SERVER['HTTP_FORWARD_HOST'] to obtain the IP address to use for 'connecting back'.

It is possible to configure multiple fallbacks by using a comma separated list of values. For example if you want to use HTTP_FORWARD_HOST first, and then also want to check REMOTE_ADDR, then you set xdebug.client_discovery_header to HTTP_FORWARD_HOST,REMOTE_ADDR.

PHP automatically prepends HTTP_, and converts - to _, for received HTTP header names. The THIS-IS-MY-HOST HTTP header is converted into $_SERVER['HTTP_THIS_IS_MY_HOST']. Therefore, the xdebug.client_discovery_header needs to be set to HTTP_THIS_IS_MY_HOST to match this.

If you have logging enabled, and set the xdebug.log_level setting to 10, then Xdebug will list every header, the header value, and the used header (if any) when attempting to find the IP address to connect back to.

Xdebug 3.2 and later no longer fall back to the $_SERVER['HTTP_X_FORWARDED_FOR'] and $_SERVER['REMOTE_ADDR'] header values by default. If you want these headers to be used as well, you specifically need to add these to the list of headers, by setting xdebug.client_discovery_header to YOUR_OWN_HEADER,HTTP_X_FORWARDED_FOR,REMOTE_ADDR.

Configures the IP address or hostname where Xdebug will attempt to connect to when initiating a debugging connection. This address should be the address of the machine where your IDE or debugging client is listening for incoming debugging connections.

Xdebug also supports a few other ways of specifying the host to connect to, but these methods are not always available. xdebug_info() shows whether the xdebug://* methods are available.

xdebug://gateway

Use the system defined network gateway.

Supported on Linux only.

xdebug://nameserver

Use the system defined private network nameserver.

Supported on Linux only, and not with distributions that use musl libc, such as Alpine Linux.

unix:///path/to/sock

A Unix domain socket.

Supported on non-Windows platforms, and only supported by select few debugging clients.

If xdebug.discover_client_host is enabled then Xdebug will only use the value of this setting in case Xdebug can not connect to an IDE using the information it obtained from HTTP headers. In that case, the value of this setting acts as a fallback only.

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

The port to which Xdebug tries to connect on the remote host. Port 9003 is the default for both Xdebug and the Command Line Debug Client. As many clients use this port number, it is best to leave this setting unchanged.

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

With this setting you configure Xdebug for use with Xdebug Cloud. It needs to match one of the tokens from your profile page.

Your IDE needs to be configured with the same token for Xdebug and your IDE to communicate through Xdebug Cloud.

In PhpStorm you can find this setting under:
File | Settings | PHP | Debug | Xdebug Cloud for Windows and Linux
PhpStorm | Preferences | PHP | Debug | Xdebug Cloud for macOS

The amount of time in milliseconds that Xdebug will wait for on an IDE to acknowledge an incoming debugging connection. The default value of 200 ms should in most cases be enough. In case you often get dropped debugging requests, perhaps because you have a high latency network, or a development box far away from your IDE, or have a slow firewall, then you can should increase this value.

Please note that increasing this value might mean that your requests seem to 'hang' in case Xdebug tries to establish a connection, but your IDE is not listening.

Linux and Windows only.

When set to "default" (the default value) or "time", then Xdebug will poll an anonymous pipe (Linux) or Named Pipe (Windows) every 25 ms for a command.

A command can currently be used to:

• request process information (time, memory usage, and entry file)
• initiate a debugging session by (trying to) connect to the IDE
• force a breakpoint

You should use the Xdebug Control tool to interact with this control socket.

When this setting is set to "off", then Xdebug will not create a control socket, and hence you can't send commands to it.

You can programatically check whether control sockets are enabled for your PHP build by calling xdebug_info() with extension-flags as category. If control sockets are available, then control-socket is returned in the resulting array. If a TSC clock is available, then tsc is also present in the resulting array.

If enabled, Xdebug will first try to connect to the client that made the HTTP request. It checks the $_SERVER['HTTP_X_FORWARDED_FOR'] and $_SERVER['REMOTE_ADDR'] variables to find out which hostname or IP address to use.

If xdebug.client_discovery_header is configured, then the $_SERVER variable with that configured name will be checked instead of the default variables.

If Xdebug can not connect to a debugging client as found in one of the HTTP headers, it will fall back to the hostname or IP address as configured by the xdebug.client_host setting.

This setting does not apply for debugging through the CLI, as the $_SERVER header variables are not available there.

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

Please note that there is no filter available, and anybody who can connect to the webserver will then be able to start a debugging session, even if their address does not match xdebug.client_host.

Controls which IDE Key Xdebug should pass on to the debugging client or proxy. The IDE Key is only important for use with the DBGp Proxy Tool, although some IDEs are incorrectly picky as to what its value is.

The default is based on the DBGP_IDEKEY environment setting. If it is not present, the default falls back to an empty string.

If this setting is set to a non-empty string, it selects its value over DBGP_IDEKEY environment variable as default value.

The internal IDE Key also gets updated through debugging session management and overrides the value of this setting as is explained in the Step Debugging documentation.

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

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:

[2693358]

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 and Flame Graphs:

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:

<init
    xmlns="urn:debugger_protocol_v1" xmlns:xdebug="https://xdebug.org/dbgp/xdebug"
    fileuri="file:///home/httpd/www.xdebug.org/html/router.php"
    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>
        <url><![CDATA[https://xdebug.org]]></url>
        <copyright><![CDATA[Copyright (c) 2002-2020 by Derick Rethans]]></copyright>
</init>

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">
   </response>

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">
           </xdebug:message>
   </response>

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: /tmp/systemd-private-ea3cfa882b4e478993e1994033fc5feb-apache.service-FfWZRg

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

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:

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.

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:

off

Nothing is enabled. Xdebug does no work besides checking whether functionality is enabled. Use this setting if you want close to 0 overhead.

develop

Enables Development Helpers including the overloaded var_dump().

coverage

Enables Code Coverage Analysis to generate code coverage reports, mainly in combination with PHPUnit.

debug

Enables Step Debugging. This can be used to step through your code while it is running, and analyse values of variables.

gcstats

Enables Garbage Collection Statistics to collect statistics about PHP's Garbage Collection Mechanism.

profile

Enables Profiling, with which you can analyse performance bottlenecks with tools like KCacheGrind.

trace

Enables the Function Trace and Flame Graphs features.

The former allows you record every function call, including arguments, variable assignment, and return value that is made during a request to a file.

The latter can be used to visualise certain performance characteristics.

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.

Step Debugging can be activated when a PHP Notice or Warning is emitted, or when a Throwable (Exception/Error) is thrown, depending on the value of this setting:

yes

Initialise a debugging session when a PHP Notice or Warning is emitted, or when a Throwable is thrown.

no

default

Do not start a debugging session upon an error situation.

This setting is independent of xdebug.start_with_request, and therefore it is not necessary to set xdebug.start_with_request=trigger.

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

yes

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.

no

The functionality does not get activated when the request starts.

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

Step Debugging and Profiling will never activate with this value.

trigger

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 or Flame Graphs), 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.

In this mode it is also possible to activate Step Debugging with xdebug_break().

default

The default value depends on xdebug.mode:

• debug: trigger

• gcstats: no

• profile: yes

• trace: trigger

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.mode=profile
xdebug.start_with_request=trigger
xdebug.trigger_value=StartProfileForMe

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:

xdebug.trigger_value=StartDebuggerForMe,StartDebuggerForYou

See also:

xdebug.start_with_request#trigger

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

Emits a breakpoint to the debug client

This function makes the debugger break on the line it is called from, as if a normal file/line breakpoint was set on this line through the debugger protocol.

If no debugging session is active yet, and xdebug.start_with_request is set to trigger, then Xdebug will attempt to start a new debugging session first.

The function returns true if a debugging session is (now) active, and the breakpoint was succesfully set. It returns false if a debugging session was not active and could not be activated.

Instructs Xdebug to establish a debugging session

This functions instructs Xdebug to try to establish a connection to a debugging client.

Normally, Xdebug tries to start a debugging session at the start of the request. If a debugging client is not listening at that point, then Xdebug does not try to re-establish a connection during the remainder of the request.

In the situation where one long running PHP process handles multiple tasks, perhaps from a queue, calling xdebug_connect_to_client() can then be used to re-try to establish a connection for a specific worker job.

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.

<?php
var_dump( 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, and all the system-specific detected features that effect Xdebug's behaviour.

If the compression 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).

If the control-socket and tsc elements are present, then Xdebug can be controlled with the Xdebug Control tool, as long as xdebug.control_socket is not set to "off".

If the control-socket flag is present, but the tsc flag is not present, then you must set xdebug.control_socket to "time" deliberately for Xdebug Control to be able to interact with Xdebug.

<?php
var_dump( xdebug_info( 'extension-flags' ) );
?>

array(1) {
  [0] =>
  string(11) "compression"
  [1] =>
  string(14) "control-socket"
  [2] =>
  string(3) "tsc"
}

Returns whether a debugging session is active

Returns true if a debugging session through DBGp is currently active with a client attached; false, if not.

Sends data to a debugging client

This function sends the contents of the $data variable over the debugging protocol to a client.

If the step debugger mode is not active, or if there is no active debugging session, this function returns false, otherwise true is returned.

A debugging client needs specific support to handle these notifications.