Lorna Mitchell has shared a tip she's found helpful when using the phan static analysis tool for finding only PHP files via a simple grep.

Phan is the PHP Analyzer for PHP 7 code. I've been using it, partly out of curiosity, and partly to look at what the implications of upgrading my various projects will be. [...] I generated my filelist.txt files with a little help from grep - by looking for all files with opening PHP tags in, and putting that list of filenames into a file.

The phan tool is still pretty young but it provides a good example of how to use the new php-ast handling to parse and analyze PHP code.