How can I find unused functions in a PHP project

You can try Sebastian Bergmann’s Dead Code Detector:

phpdcd is a Dead Code Detector (DCD) for PHP code. It scans a PHP project for all declared functions and methods and reports those as being “dead code” that are not called at least once.

Source: https://github.com/sebastianbergmann/phpdcd

Note that it’s a static code analyzer, so it might give false positives for methods that only called dynamically, e.g. it cannot detect $foo = 'fn'; $foo();

You can install it via PEAR:

pear install phpunit/phpdcd-beta

After that you can use with the following options:

Usage: phpdcd [switches] <directory|file> ...

--recursive Report code as dead if it is only called by dead code.

--exclude <dir> Exclude <dir> from code analysis.
--suffixes <suffix> A comma-separated list of file suffixes to check.

--help Prints this usage information.
--version Prints the version and exits.

--verbose Print progress bar.

More tools:

• https://phpqa.io/

Note: as per the repository notice, this project is no longer maintained and its repository is only kept for archival purposes. So your mileage may vary.