$Entrada = strtotime( '2010-05-26 08:00' );
    $Saida   = strtotime( '2010-05-26 11:15' );
    $Diferenca = $Saida - $Entrada;

    $hora = $Diferenca/3600;
    $min  = $Diferenca/60%60;

    printf( '%d:%d', $hora, $min );

$Entrada = strtotime( '2010-05-26 08:00' );
    $Saida   = strtotime( '2010-05-26 11:15' );
    $Diferenca = $Saida - $Entrada;

    $hora = $Diferenca/3600;
    $min  = $Diferenca/60%60;

    $Resultado = printf( '%d:%d', $hora, $min );
    echo $Resultado;