For the past 3 years now I have been using phpDesigner with great pleasure it has everything you need as a developer (don't remember their website but try search for phpDesigner at Google)...
I had only experience coding HTML/CSS but wanted to expand this to PHP also so I started 3 years ago searching the web for a good PHP editor / PHP IDE and found phpDesigner 🙂
It helped me learn the php language and today I actually code websites in HTML/PHP for other people!!
Update: Although phpDesigner is not an open-source php ide it is still very recommended and has a low price tag compared to zend etc.
Magnus