There are many PHP IDE's (integrated development environment) available, both free and commerical. One of the best is phpDesigner.

What is an IDE?

An integrated development environment is a software application that provides comprehensive facilities to computer programmers for software development. An IDE typically consists of:

• a source code editor
• an interpreter
• a debugger
• integration of a version control system (such as Subversion, CVS, or GIT)

IDE's are meant to simplify a programmer's code and to enhance thir development and productivity.