PhpDesigner is one of the most popular PHP IDE's on the market. At 39 Euros it is an absolute steal compared with the more expensive IDE's, like Zend Studio ($299). It is packed with features that should speed up your development time.

PhpDesigner  is all you need for your web-development.
Rapid fast and full-featured PHP editor and PHP IDE with built-in HTML, CSS and JavaScript editors and FTP/SFTP-client. Using it you can create, edit, debug, analyze and publish PHP, HTML, CSS and JavaScript.
 Boost productivity with tons of time-saving features to speed up your development
 Work with jQuery, Ext JS, YUI, Dojo, MooTools, Prototype and all PHP frameworks

Live parsing: The live parsing is a great help. As you type your code, phpDesigner is parsing and it and checking it for syntax errors. It reports any errors on the bottom of the screen as you type.

Function IntelliSense and descriptions: As you begin typing a PHP function name, the IntelliSense will give you a list of available functions for the letters that you have already typed. It will also display a short description for that function, which is very handy if you are not sure of the exact description to chose.

If you have are returning to a script, you can click on a PHP function name and the description will appear.

Interface: The interface is visually pleasant and fairly well laid out.

Project management tools: phpDesigner comes equipped with project manager tools like a debugging tracker.

There are loads of features with phpDesigner, as listed on their website. The above is just a short list of my favorites.

Link to the official website: http://www.mpsoftware.dk/phpdesigner.php