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
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:
Syntax highlighting for PHP, HTML, CSS, Javascript etc.
Auto indentation
Beautify PHP using PHP_Beautifier
Line numbering
etc.
Geany
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:
Syntax highlighting
Code folding
Symbol name auto-completion
Auto-closing of XML and HTML tags
Simple project management
Plugin interface
etc.
Komodo Edit
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:
Autocompletion
Templates
Syntax highlighting
Syntax Checking
Syntax Coloring
Code Snippets
Code Folding
Project explorer
Navigation
Standard Editing Features
Search Highlighting
Editor Hyperlinking
and many more
Some other useful PHP IDEs could be find in the list below: