Of particular interest to us has been a good, working free/FLOSS PHP debugging plug-in (using either the XDebug or DBG Apache/PHP plugins).
At the top of the list of free tools come Eclipse PHP Developer Tools (aka Eclipse PDT) and NetBeans for PHP & they appear to be well-integrated and work pretty much as advertised. I say 'pretty much' b/c as with a lot of software that's under active development, it's still in beta & prone to issues.
The premier tool is the non-free Zend Studio, but be prepared to fork over $300 to Zend. For a first-rate dev tool, $300 ain't bad, but if free is good enough (for the time being...) then $0 is good too.
From the top of my head....
NetBeans (current) - Works on SuSE Linux 11.1 / 11.3, but prone to locking up the system.
Eclipse PDT (current) - Debugger support requires Zend Server Community Edition (CE) & xDebug. Only xDebug Windows binaries or actual source code are available at the xdebug website. To get Linux binaries requires a visit to one's own native repositories or compiling it from source.
EasyEclipse (defunct but available) - Windows only, debugger support?
Quanta+ (current, but PHP support ending?) Linux only. PHP support is in flux (curtailed). See also KDevelop4....
KDevelop4 (current) Linux only. PHP might be supported but under development? Did not work as expected on i586 SuSE Linux.
Apparently there is no good consolidated resource of all the tools that are available, you have to just be willing to Google away & sometimes just wait to stumble upon some unexpected tools and projects. Many electrons have been inconvenienced in discussion regarding PHP IDE's & debuggers but there's no one single clearing house of information on this. It's all rather, well, chaotic and seemingly word-of-mouth (search-of-google).
I'll list all the tools I have found in an upcoming post, the list is quite large and yet probably by no means exhaustive. There are a variety of dead opensource projects littering the landscape, like a Sri university effort to create a working PHP IDE with an integrated debugger and so on.
