If you supply 3 arguments and the 2nd is an array, strtr will search the "A" from "Array" (because you're treating it as a scalar string) and replace it with the 3rd argument:
strtr('Analogy', array('x'=>'y'), '_'); //'_nalogy'
so in reality the above code has the same affect as:
strtr('Analogy', 'A' , '_');