diferencia de horas php con strtotime

scratch.php

$hora_ini = "08:15";
    $hora_fin = "09:30";
    $fecha = date("Y-m-d");

    $ts_fin = strtotime($fecha." ".$hora_fin);
    $ts_ini = strtotime($fecha." ".$hora_ini);
    echo "TS_INI: ".$ts_ini."<br>";
    echo "TS_FIN: ".$ts_fin."<br>";

    echo "DIFF: ".($ts_fin-$ts_ini)/3600;