Calcular Diferença de Horas entre Datas

calc.method

static function getIntervalUnits($interval, $unit)
{
    // Day
    $total = $interval->format('%a');
    if ($unit == 'days')
        return $total;
    //hour
    $total = ($total * 24) + ($interval->h );
    if ($unit == 'hours')
        return $total;
    //min
    $total = ($total * 60) + ($interval->i );
    if ($unit == 'minutes')
        return $total;  
    //sec
    $total = ($total * 60) + ($interval->s );
    if ($unit == 'seconds')
        return $total;  

    return false;
}

calculo

$dataEntrada = '18/07/2013 15:00';
$dataSaida = '23/07/2013 23:00';

$format = 'd/m/Y H:i';

$timeZone = new DateTimeZone('America/Sao_Paulo');

$entrada = DateTime::createFromFormat($format, $dataEntrada, $timeZone);
// echo $entrada->format('Y-m-d');

$saida = DateTime::createFromFormat($format, $dataSaida, $timeZone);
// echo $saida->format('Y-m-d');

$interval = $entrada->diff($saida);
echo $interval->format('%a dias and %h horas') . '<br />';

$diasTotal	= $interval->days;
$horas		= $interval->h;
$horasTotal = ($diasTotal * 24) + $horas;

echo 'Total de ' . $horasTotal . ' horas.';

echo '<br />';

// var_dump($interval);