Modify project nature in eclipse

As said here, you can modify the .project file to add your builders, and the nature of the project.

This works for my eclipse PDT 3.3.2

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
 <name>symfony</name>
 <comment></comment>
 <projects>
 </projects>
 <buildSpec>
  <buildCommand>
   <name>org.eclipse.wst.jsdt.core.javascriptValidator</name>
   <arguments>
   </arguments>
  </buildCommand>
  <buildCommand>
   <name>org.eclipse.php.core.PhpIncrementalProjectBuilder</name>
   <arguments>
   </arguments>
  </buildCommand>
  <buildCommand>
   <name>org.eclipse.php.core.ValidationManagerWrapper</name>
   <arguments>
   </arguments>
  </buildCommand>
 </buildSpec>
 <natures>
  <nature>org.eclipse.php.core.PHPNature</nature>
  <nature>org.eclipse.wst.jsdt.core.jsNature</nature>
 </natures>
</projectDescription>