Is x a prime number?

prime-number.php

<?php 
function isPrime($number) 
{
    // the only even prime number is 2
    if (($number % 2 == 0 and $number != 2) or $number == 1) { 
        return false;
    }   

    $finish = floor($number / 2); // the higher denominator of any number is its half

    $denom = 3;
    while ($denom <= $finish) {
        if ($number % $denom == 0) { 
            return false;
        }
        $denom += 2; // next odd number, odd numbers can't be divided by even ones
    }

    return true;
}

/*----------------------------*/

$primes = array_filter(range(1, 100), 'isPrime');

echo 
'Prime numbers between 1 and 100: '.implode(', ', $primes);