Date difference is not working

Question

$currentDate = date('Y-m-d H:i:s');
    $lastDate = $data[0]['inputTime'];
   <?php    
        echo "<br>Current Date ".$currentDate;
        echo "<br>Last Date ".$lastDate;

        $diff = round(abs($currentDate - $lastDate) / 60,2);
?>

its not working i am getting $lastdate from mysql and data type is DateTime but its not working please help if there is another way please guide me i am testing it on live domain its working fine on localhost although