First of all: the used algorithm passed '1' for a prime while it isn't! A prime should have two and only two different
Secondly, you could speed things up a little more: you only need to check all integer numbers up to ceil( $input / 2 ) because of the commutative properties of integers (2*3 == 3*2).
Furthermore, prime are always (except for '2', so almost always) odd numbers, so you could skip them in your test for even more speed improvement!
"A chicken is an egg's way of producing more eggs."