Using usort in php with a class private function

Question

ok using usort with a function is not so complicated

This is what i had before in my linear code

function merchantSort($a,$b){
    return ....// stuff;
}

$array = array('..','..','..');

to sort i simply do

usort($array,"merchantSort");

Now we are upgrading the code and removing all global functions and putting them in their appropriate place. Now all the code is in a class and i can't figure out how to use the usort function to sort the array with the parameter that is an object method instead of a simple function

class ClassName {
   ...

   private function merchantSort($a,$b) {
       return ...// the sort
   }

   public function doSomeWork() {
   ...
       $array = $this->someThingThatReturnAnArray();
       usort($array,'$this->merchantSort'); // ??? this is the part i can't figure out
   ...

   }
}

The question is how do i call an object method inside the usort() function

Answer 1

Make your sort function static:

private static function merchantSort($a,$b) {
       return ...// the sort
}

And use an array for the second parameter:

$array = $this->someThingThatReturnAnArray();
usort($array, array('ClassName','merchantSort'));