PHP Coder is an IDE (Integrated Development Environment) especially developed for PHP programmers.
Through tight integration of the PHP interpreter and the PHP documentation, PHP Coder gives you a time-saving Development Interface. Features are Full Integration of the PHP Interpreter and the PHP documentation, Integrated Preview window, Full Syntax Highlighting for HTML and PHP, AutoComplete for frequently used text constructs, Project manager, Tag Buttons and all the standard editing functions like almost unlimited undo, copy and paste, searching, ...
And it is quite small compared to other language's IDEs and loads pretty fast.
And the best, it's FREE.

Homepage: http://www.phpide.com/
Download: PHP Coder R2 FP3 ( 983KB )