Xdebug can show which functions consume time or memory in a PHP script. Enable its profile mode for the PHP runtime you want to measure, write the resulting Cachegrind-compatible file to a writable directory, then inspect it in a compatible viewer. For occasional checks, use trigger startup so ordinary requests are not profiled.
1. Confirm which PHP runtime runs your script
CLI PHP and the PHP runtime used by a web server may load different configuration files. First identify the configuration for the specific process you intend to profile: Xdebug recommends php --ini for CLI or a phpinfo() page for a web runtime. See Xdebug’s installation documentation.
Do not assume that a setting added to CLI’s php.ini affects web requests, or the reverse. Verify the active configuration for the target runtime before troubleshooting profiling.
2. Enable profiling, preferably only when needed
Set xdebug.mode=profile in the applicable PHP configuration. With profile mode, the default xdebug.start_with_request behavior is yes, which starts profiling automatically. That can create files for every request handled by that runtime.
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Profile selected requests with a trigger
For targeted profiling, configure trigger startup and an output directory:
xdebug.mode=profile
xdebug.start_with_request=trigger
xdebug.output_dir=/tmp/xdebug-profilesWhen startup is set to trigger, Xdebug starts profiling when it finds XDEBUG_TRIGGER in an environment variable, GET or POST parameter, or cookie. For example, a CLI process can receive the environment trigger like this:
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XDEBUG_TRIGGER=1 php script.phpIf xdebug.trigger_value is configured, the trigger must match its required value. For a web request, send XDEBUG_TRIGGER through a supported channel, such as a query parameter or cookie, subject to that setting. The current trigger name is XDEBUG_TRIGGER; refer to Xdebug’s installation documentation for details.
Use XDEBUG_MODE for a one-off CLI run
You can select profile mode for a CLI process without changing the configured xdebug.mode value:
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XDEBUG_MODE=profile php script.phpXDEBUG_MODE overrides the configured mode for that process. In PHP-FPM, however, environment variables may not reach PHP: the clear_env setting is on by default. If Xdebug appears to ignore XDEBUG_MODE for web requests, check PHP-FPM’s environment filtering and explicitly allow the variable or adjust that setting. Xdebug documents these configuration details in its settings reference.
3. Find and manage the profile output
Xdebug writes profiler files to xdebug.output_dir, which defaults to /tmp. The PHP process user must be able to write to the chosen directory. By default, output filenames begin with cachegrind.out. and end with the PHP or Apache process ID; xdebug.profiler_output_name can change the naming format. See Xdebug’s settings reference.
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- Check that the configured output directory exists and is writable by the relevant PHP process.
- Watch available disk space. Xdebug warns that profiles for complex scripts can be enormous.
- For a profiled HTTP request, look for the
X-Xdebug-Profile-Filenameresponse header; it can identify the file written for that request.
Automatic startup can generate many files because every request is profiled. Trigger startup limits capture to requests where you deliberately send the trigger.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.4. Open the Cachegrind-compatible file
Xdebug’s profiler outputs data in a Cachegrind-compatible format. The Xdebug profiling documentation names several ways to inspect it:
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| Tool | Interface described by Xdebug |
|---|---|
| KCacheGrind | Desktop visualization; identified as a Linux/KDE option. |
| QCacheGrind | Desktop visualization; identified as an option for Windows and available for macOS through Homebrew. |
| Webgrind | Web-based frontend. |
ct_annotate |
ASCII output. |
Those platform and distribution details can change, so verify current packaging before installing a tool. Choose a desktop interface, web frontend, or text output based on how you want to navigate results; check that your chosen tool supports the generated file and any compression settings in use. The documentation establishes Cachegrind compatibility but does not rank these tools or compare all their current features.
5. Use the profile to investigate bottlenecks
In the viewer, look for functions with high costs and examine their call relationships to understand where work is occurring. Profiling data can also help investigate memory use. Treat a profile as evidence about the workload you captured: it points to areas worth examining, but does not itself promise a particular speedup.
Quick Recap
- Capture a representative script run or request with the profile trigger.
- Open its output file in a compatible viewer and identify a costly function or call path.
- Make a focused change, then profile the same representative workload again to assess the result.
Troubleshooting missing or unusable profiles
- No file appears: confirm profile mode is active for the intended runtime, then check
xdebug.output_dirand write permissions. - CLI profiling works but web profiling does not, or vice versa: inspect the active configuration separately for each runtime.
XDEBUG_MODEhas no effect in PHP-FPM: check whether its environment filtering prevents the variable from reaching PHP;clear_envdefaults to on.- Too many or unexpectedly large files: check whether startup is set to
yes, switch to trigger startup for selective captures, and monitor disk capacity. - A viewer will not open the file: verify that it supports the generated format and check whether compression settings affect its ability to read the output.
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