Eclipse PDT here. Why? Well, it's easy to use, highly customizable, and most important, very extensible. It has support for everything you'd need: php editor, syntax highlighting, code completion, error reporting as you type, support for debugging, code outline, snippets, html + javascript + css editor (provided by aptana plugin), database development & debugging, regex builder (extension) and so many more (if you really need more than all that)...

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