PHPEverywhere has a cool link to a PHP debugger that promises several things, including being able to:

• debug your php program using Step, Goto, Run command,
• view source of program, current line,
• GET a variable's value in each step,
• even more, you can MODIFY the value of a variable when debugging, without restart.
• use a watch window to specify variables that you want to WATCH, along with debugging your php program

Pretty cool stuff...the only problem is, it's not the free stuff we're all so used to. $30 isn't bad, but still more than $0. Check out phpdebug.com for more info.