Create a static function for sort inside class:
private static function merchantSort($a,$b) {
    return …// the sort
}
Call the sort function using below code:
$array = $this->someThingThatReturnAnArray();
usort($array, array(‘ClassName’,’merchantSort’));
$array = $this->someThingThatReturnAnArray();
usort($array, array(‘ClassName’,’merchantSort’));