Description:
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http://www.php.net/manual/en/language.operators.arithmetic.php
http://www.php.net/manual/fr/language.operators.arithmetic.php

English :
"The division operator ("/") returns a float value unless the two operands are integers (or strings that get converted to integers) and the numbers are evenly divisible, in which case an integer value will be returned."

French :
"L'op?rateur de division ("/") retourne une valeur enti?re (le r?sultat d'une division enti?re) si les deux op?randes sont entiers (ou bien des cha?nes converties en entier)."

The french version doesn't talk about "evenly divisible numbers", so we can think that the operator really makes integer divisions

15 (int) / 4 (int) = 3 (int)

which is not the case

15 (int) / 4 (int) = 3.75 (float)