Use PHP Eclipse:
plus points: free for any use, no license issues at all
stable once setup correctly
I use PHPEclipse for PHP debugging - stepping through, watching variables etc.
use eclipse 3.0.2 not 3.1
read all the steps carefully and follow them meticulously
(3) contains the maximum detailed instructions
lets say you will take minimum one day to set it all up - becasue you are new to Eclipse - but it is all worth it if you get is right - mine is working ever so smoothly since the past month.
Warning: eclipse takes time to get used to - you do not find menu options in places wher you find normal menus on windows - instead you will get them at right -clicks and popup menus - but eclipse is consistent - Eclipse for Java is the same as Eclipse for C is the same as Eclipse for PHP is the same as EClipse for HTML -just that is different from MS software.
Also, you need to start a project to do anything at all -> File ->new ->project etc.
Eclipse is really easy once you get used to it. try it if you have the time.
If you have problems, post here, though I cannot guarantee timely replies.
However, if you do not want step-debugging then echo statements here and there will be helpful