Run php in lint-mode from the command line to validate syntax without execution:

php -l FILENAME

Higher-level static analyzers include:

Lower-level analyzers include:

Runtime analyzers, which are more useful for some things due to PHPs dynamic nature, include:

The documentation libraries phpdoc and doxygen perform a kind of code analysis. Doxygen, for example, can be configured to render nice inheritance graphs with graphviz.

Another option is xhprof, which is similar to xdebug, but lighter, making it suitable for production servers. The tool includes a PHP-based interface.