If you are new to Eclipse PDT and find that code assist or the PHP Manual (Shift+F2 or Open PHP Manual) are not working for a particular project, check that you have a file named .buildpath in the root of your project.
If there is nothing there, try creating a new project (File / New / PHP Project) grab the file from there and copy it into your existing project.
Alternatively just paste in the following:
<?xml version="1.0" encoding="UTF-8"?>
<buildpathentry kind="src" path=""/>
<buildpathentry kind="con" path="org.eclipse.php.core.LANGUAGE"/>