Prime Palindrome

Prime Palindrome

<?php
/**
 * Write a program to determine the biggest prime palindrome under 1000.

    INPUT SAMPLE:
    There is no input for this program.

    OUTPUT SAMPLE:
    Your program should print the largest palindrome on stdout, i.e.
    929
 * */

// first create a sieve containing all the prime numbers between
// 1 and 1000
$highEnd = 1000;
$maxCheck = sqrt($highEnd);
$sieve = array_fill(0,1000, true);

for ($i = 2; $i<= $maxCheck; $i++) {
    if ($sieve[$i] == true) {
        for ($j = $i * $i; $j < $highEnd; $j += $i) {
            $sieve[$j] = false;
            }
    }
}
// filter out all the non-primes
//$sieve = array_filter($sieve);

// starting at the largest prime in the sample
// loop backward until a palindrome is encountered
do {
    end($sieve);
    $key = (string) key($sieve);
    $value = array_pop($sieve);
    $length = strlen($key) -1;
    $found = false;
    if ($value) {
        $found = true;
        for ($i=0; $i< $length; $i++) {
            if ($key[$i] != $key[$length-$i]) {
                $found = false;
            }
        }
    }

} while (! $found);

echo $key;