如何在两个日期时间之间做出时间差？

I want to return the difference between two datetime objects.

I tried this but is not working. Whats wrong?

<?php //php 7.0.8

    $a='2018-12-05 09:00:00';
    $b='2018-12-05 15:00:00';

    $time1 = new DateTime($a);
    $time2 = new DateTime($b);

    $interval = $time1->diff($time2);
    echo $interval->format('H:i');
?>

Each format character must be prefixed by a percent sign (%).

echo $interval->format('%H:%i');