How to add a PsiReferenceContributor for an existing language?

I'm having trouble understanding the correct way to implement a reference provider. I'm not creating a custom language plugin, so a lot of possible solutions i found to my problem don't apply here. So far i have:

plugin.xml

<extensions defaultExtensionNs="com.intellij">
     <psi.referenceContributor language="PHP" implementation="my.plugin.MyReferenceContributor" />
</extensions>

MyReferenceContributor.java

public class MyReferenceContributor extends PsiReferenceContributor
{
    @Override
    public void registerReferenceProviders(PsiReferenceRegistrar registrar)
    {
        // this doesn't get executed at all
        registrar.registerReferenceProvider(PlatformPatterns.psiElement(PhpNamedElement.class), new MyReferenceProvider());
    }
}

MyReferenceProvider.java

public class MyReferenceProvider extends PsiReferenceProvider
{
    @NotNull
    @Override
    public PsiReference[] getReferencesByElement(@NotNull final PsiElement element, @NotNull final ProcessingContext context)
    {
        // this doesn't execute
        System.out.println(((PhpNamedElement) element).getName());

        return new PsiReference[0];
    }
}

Is this the correct way to do this? Why isn't my provider registered?