phpdoc is a program that aims to do for PHP developers what Javadoc does for java developers. It is a script that parses PHP class definitions, extracting function and variable names, as well as comments, and dumping this data into a database. The script itself is built in Perl, but PHP is used for search and display. Search is powered by SearchForms, display by EnzymeTemplates, both of which are part of the Enzyme open-source project.
phpdoc is very similar to Gareth Watts' PHPXref.