PHP match specific letters of sentence and get only the words

I'm tiring to write a PHP code for get matched words by using letters, its not the same strpos.

 ex
    $string="How are you robin ?";
    $searchletters="yo";


 in this  "$string" when i search "$searchletters" i need to get 
 the word "you" which have you letters, 

anyone have a idea how to implement this with PHP. Thank You