Non so che dirti, facendo:

codice:
$diffe=date_diff($ora_fine,$ora_inizio);
$diffe->format('%H:%I');
$diffe contiene:

codice:
(
    [y] => 0
    [m] => 0
    [d] => 1
    [h] => 16
    [i] => 5
    [s] => 0
    [f] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 1
    [days] => 1
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)