Netbeans is a powerful IDE (integrated development environment) that supports many languages. It got its start as a Java IDE, and in fact its written in Java, so it runs on Windows, Linux, and Mac.
Netbeans is open source and free of licensing costs.
At the time of this writing, I am using Netbeans 6.8 for PHP development.
PHP is an interpreted language that can be developed and maintained with any bare bones text editor or notepad application. While you can work this way, you're losing a lot of productivity in doing so.
Here are just a few reasons why I like Netbeans:
Syntax coloring. At a glance you will be able to distinguish comments from code, from strings, etc. This gets your eye right to what you're looking for and allows you to filter out the noise.
SVN integration. SVN (subversion) is my version control system of choice right now. The Netbeans project explorer will color code the source file icons according to their state with respect to SVN: unchanged, modified, or new. The diff is very pretty and makes it very easy to see where code has been inserted, modified, or deleted,
Smart code navigation. You can right-click on a method and select "Go to Declaration" and Netbeans will open the right source file and position you where the method is defined. In any non-trivial PHP application you will find that there are many folders, often with many instances of files called controller.php or view.php, and the like. Not having to manually flip through a half-dozen open windows showing different controller.php files saves my sanity.
Interactive debugging!! Netbeans has support for xdebug which allows one to set breakpoints, single-step through code, examine call stacks, and variables in scope. Problems that might take you days to discover by reading code and sprinkling echo statements often can be solved in hours or minutes with the use of an interactive debugger. This feature alone puts Netbeans into my toolbox.
There are plenty of other free and commercial IDE's out there. Not intending to exclude, I would say that Eclipse is the closest competitor in the free, open source IDE space. It is well-established and has a huge user base. I just prefer Netbeans.