Assuming you mean safe in terms of SQL injection or XSS attacks, then probably yes. Casting to an
int only makes sure the value is an integer. An integer is not usually dangerous in any context. It does not guarantee the safety of the integer's value though. It may be
0, which may or may not have a special meaning in your code, for example when comparing to
false. Or it may be negative, which, again, may or may not have any side effects in your code.
"Safety" isn't an absolute thing. The string
"1 = 1; DROP TABLE users" by itself is pretty safe, too. It just depends on the context you're using it in. Just the same, a
0 is perfectly safe until your code includes
if (!$number) deleteAllUsers();.