I use to use PHPDESIGNER v7. But now I just used "notepad2", and if I need help I just refer to a online source or one of the 20 books I have
Don't pay for over bloated WYSIWYG editor such as Dreamweaver. Utilizing simple editors like Notepad++ and Sublime Text (evaluation usage is free but for continuous usage you pay $70 Sublime Text 3) and your browser window you can achieve the same pixel prefect results if you're aware how each popular browser may break the code for your website or application you're building.
I use Coda on my Mac, I'd say it's the best I've used on so far for a Mac. If your using a Windows computer, I'd suggest Notepad++.
I use UltraEdit - excellent editor, check my review here: http://www.webthesmartway.com/2013/02/ultraedit-text-editor-for-windows/
I use a text editor Vim. Is similar to Notepad + +, but I think it's still much better than Notepad + + and ser can define personalized key mappings.
It depends on how much you are familiar with the interface, every one should prefer their own editor they are using as i have ConText, i am inspired because the interface is pretty friendly and lite.
I quite like Notepad++ and always keep it in my arsenal, however I do tend to find myself using PHPDesigner alot, I like how easy it is to add ftp and remotely edit files.
I'm surprised more people haven't said Zend Studio, but I would definitely recommend it as the best. For a free simple alternative, Notepad++ is great, as has been mentioned here several times.
I'm pretty surprised that no one has mentioned PHPStorm (by JetBrains) - so far it's the smartest PHP IDE that i've seen. Text editors like sublime/notepad++/ultraedit/... are good except they can't offer debugging, refactoring, code inspection, class/methods suggestion, etc.