There are so many IDE's that based on Eclipse platform, and countless projects have been crafted in Eclipse IDEs. For PHP development this is Eclipse PDT (PHP Development Tools) project.
Eclipse PDT was developed jointly by IBM and Zend, although participators come from beyond those two organisations. Not long ago, Aptana (whose Aptana Studio supports PHP) has begun significant contributions to the PDT project.
Eclipse PDT is available for Windows, Linux, and Mac OS X. Windows and Mac OS X versions can be had in 32-bit or 64-bit variants.