It's not really an IDE, as it is only an editor, but I use notepad++. I then just have an apache server set up with PHP and MySQL. With this all you need to have is a file editor to edit the php files in the http root. Works great for me!
I work now in Codelobster PHP Edition. It's very comfortable and free PHP, HTML, CSS, JavaScript editor. The last version become Portable, that allows to install it on the flash drive and use it on any computer Want to mark the presence of PHP debugger, that make this editor valuable PHP IDE. +it have special plugins for autocomplete and context help for Drupal, Joomla, JQuery, Smarty, WordPress, Facebook, CakePHP and CodeIgniter. http://www.codelobster.com
I'm using Komodo Edit 6, it's very comfortable for me, already integrated with HTML, CSS, PHP etc library.. also it will detect your function if you include it, even on other file..