<?php

function dateDifference($date_1 , $date_2 , $differenceFormat = '%h' )
{
    $datetime1 = date_create($date_1);
    $datetime2 = date_create($date_2);
    
    $interval = date_diff($datetime1, $datetime2);
    
    return $interval->format($differenceFormat);
    
}


echo dateDifference('1999-01-01 13:30:00', '1999-01-01 15:30:00');