Facebook PHP SDK - is user "liking" my app?

I have two things:
- how to do: using PHP SDK for Facebook check that user is liking my app?

I need this because client want it.

 $isLike = /* Code to check this */
 
 if ($isLike){
 
 //if user like my app
 }else{
 
  //if not
 
  include 'generate.php';
 }
 

And what I should ask this question in "valid" english?

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