return backward read primes

backward_read_primes.php

<?php

/**
 * return backward read primes
 *
 * Find all Backwards Read Primes between two positive given numbers
 * (both inclusive), the second one being greater than the first one.
 * The resulting array or the resulting string will be ordered
 * following the natural order of the prime numbers.
 *
 * @param int $start
 * @param int $end
 * @return array $primes
 * @author Phan Vu Hoang <vu.hoang-phan@ekino.com>
 */
function backwardsPrime($start, $end) {
  $res = [];
  for ($i=$start; $i <=$end ; $i++) {
    if(isPrimeNum($i) && isBwpNum($i)) {
      //$res[] = $i;
      array_push($res,$i);
    }
  }
  return $res;
}

/**
 * check whether a number is prime
 */
function isPrimeNum($number) {
  for ($i=2; $i < $number ; $i++) {
    if($number % $i == 0)
      return false;
  }

  return true;
}

/**
 * check whether a number is backward read price
 *
 * @return boolean
 */
function isBwpNum($number) {
  $number_nd = intval(strrev((string)$number));

  return isPrimeNum($number_nd);
}

$primes = backwardsPrime(7000, 7100);
echo '<pre>';
print_r($primes);
echo '</pre>';