- substr: documentation ( source)
<?php
function isPrime($num) {
for ($i = 2; $i < $num; $i++) {
if ($num % $i == 0) {
return false;
}
}
return true;
}
function firstAndLastDigitEqual($num, $digit) {
return substr($num, 0, 1) == (string)$digit && substr($num, -1) == (string)$digit;
}
$foundPrimes = 0;
for ($i = 1; true; $i++) {
if (firstAndLastDigitEqual($i, 3) && isPrime($i)) {
if (++$foundPrimes == 333) {
echo $i;
break;
}
}
}