PHP Coder is an Integrated Development Environment specially made for PHP developers.
Through tight integration of the PHP interpreter and the PHP documentation, PHP Coder gives you a time-saving Development Interface. Features are Complete Integration of the PHP Interpreter and the PHP documentation, Integrated Preview window, Full Syntax Highlighting for HTML and PHP, Auto-Complete for frequently used text constructions, Project manager, Tag Buttons and all the main editing functions like almost unlimited undo, copy and paste, searching, ...
And it is quite small compared to other language's IDEs and loads so fast.

Link to the official website: http://www.phpide.com/phpcoder.htm