getting a percentage using number_format()

[micmac]

Hi all,
If i have the following:

number_format($movie_old_price-$movie_current_price, 2, ".", ",")

how can i get the difference as a percentage?
For example if $movie_old_price = 39.95 and $movie_current_price = 29.95 what is the difference as a percentage?

Cheers,
micmac

[tasistro]

($movie_old_price/$movie_current_price-1)*100