Find if an integer is a prime number

The algorithm can be improved further by observing that all primes are of the form 6k ± 1, with the exception of 2 and 3. This is because all integers can be expressed as (6k + i) for some integer k and for i = ?1, 0, 1, 2, 3, or 4; 2 divides (6k + 0), (6k + 2), (6k + 4); and 3 divides (6k + 3). So a more efficient method is to test if n is divisible by 2 or 3, then to check through all the numbers of form 6k ± 1.

function isPrime($n)
{
    // Corner cases
    if ($n <= 1)  return false;
    if ($n <= 3)  return true;

    // This is checked so that we can skip
    // middle five numbers in below loop
    if ($n % 2 == 0 || $n % 3 == 0) return false;

    for ($i=5; $i*$i<=$n; $i=$i+6) {
        if ($n % $i == 0 || $n % ($i+2) == 0)
           return false;
    }

    return true;
}