I use Aptana with PHP and the content assist generally works. I will try to give some examples in which ways it doesn't work for me:
Inheritance doesn't work at all if you use parent:: functionality. If you have Class A which extends Class B, then in Class A the parent:: content assist doesn't work. Additionally the "Click to follow" functionality doesn't work (if you click with the ctrl/cmd key on parent:: there is no link to the according class).
Inheritance also doesn't work if you get objects over method calls.
Content assist works for:
but does not assist for:
I don't get the possible methods/vars from the AnotherObject.
The method getAnotherObject() in the $obj class is defined with phpdoc-style to have a return type to get content assist functionality.