Yesterday, I migrated from PHPEclipse (and Eclipse 3.1) to the PHP IDE (and Eclipse 3.2). After importing the projects to the PHP IDE I noticed that the Outline view (which lists class variables, methods, etc.) and the auto-completion didn’t work. Hm. The problem was that the .project file still contained references to PHPEclipse. So I had to replace:

<buildSpec>
    <buildCommand>
        <name>net.sourceforge.phpeclipse.parserbuilder</name>
        <arguments>
        </arguments>
    </buildCommand>
</buildSpec>
<natures>
    <nature>net.sourceforge.phpeclipse.phpnature</nature>
</natures>

with:

<buildSpec>
    <buildCommand>
        <name>org.eclipse.wst.validation.validationbuilder</name>
        <arguments>
        </arguments>
    </buildCommand>
    <buildCommand>
        <name>org.eclipse.php.core.PhpIncrementalProjectBuilder</name>
        <arguments>
        </arguments>
    </buildCommand>
</buildSpec>
<natures>
    <nature>org.eclipse.php.core.PHPNature</nature>
</natures>

And voilà, the project was now recognized as a PHP project, and both Outline view and auto-completion worked.

Thanks to Seva Lapsha for this tip!