If you are unable to find any good and free PHP IDE for Ubuntu (Linux) then this blog should help you in finding some.
gEdit is a built in software in Ubuntu, which is normally used as a text editor for your text files. But this could also be used for other languages like PHP etc. It has a good interface when talking about PHP, which will show you colorful environment and some other good features.
Some of the gEdit features include:
Beautify PHP using PHP_Beautifier
If you think gEdit is not the one, and you want some advance features with light weighted software, then Geany could provide you some extra features which gEdit does not have. Projects could easily be managed on Geany, and you can install some other plugins for Geany as well.
Some of the Geany features include:
Symbol name auto-completion
Auto-closing of XML and HTML tags
Simple project management
Komodo Edit is little heavier comparing to gEdit and Geany, but it really has good functionality like Autocompletion, Find in Files, Projects etc.
Some of the Komodo Edit features include:
Standard Editing Features
and many more
Some other useful PHP IDEs could be find in the list below: