In a new post to his blog Klaus Graefensteiner takes a look at two ways to test for prime numbers both with Sieve and from a file.
In PHP is really no ideal way to test large integers and determine whether they are prime numbers or not. The most popular algorithm for finding prime numbers is a memory and resource hog. It is called The Sieve of Eratosthenes.
You can get more of an idea on this method in this video and in this example from Wikipedia. He includes the full source for his solution that includes methods like isInteger, isPositive, isPerfectSquare and isPalindromicPrime. The script is also available for download.