There are several IDEs available for different platforms, most of which are commercial, but there are some free ones, too. lists some of the most popular PHP IDEs, Eclipse PDT Komodo IDE NetBeans phpDesigner PHPEclipse PhpED PHPEdit Zend Studio The most good which is suitable is PhpEd. It has alot of feature which a true Php IDE should have.
Try aptana studio; IMHO get the older 1.x version with the php plugin and not the 2.x version with PDT. I have not used the version 2.x, but there are many forum posts saying that php on 2.x is terrible, but the 1.x version works very well and since I can't afford downtime i have not upgraded . Has Win and Lin versions.
I too love Aptana studio. Like GNetCoder said, use the older version with the php plugin. The sysntax highlighting and code completion works great.