Function to check membership of an IPv4 address in an IPv4 CIDR block.

ip_in_block.php

<?php

/**
 * Check whether a IPv4 CIDR notation block contains an IP address.
 *
 * @param $cidr string
 *     An IPv4 CIDR block like "192.168.56.0/24".
 * @param $ip string
 *     An IPv4 address like "192.168.56.101".
 * @return bool
 *     TRUE iff $ip is within the $cidr block.
 */
function ip_in_block($cidr, $ip) {
  list($addr, $length) = explode('/', $cidr, 2);

  $length = $length * 1;
  $zeros = 32 - $length;
  $mask = (0xFFFFFFFF >> $zeros) << $zeros;  

  $addr = ip2long($addr);
  $ip = ip2long($ip);

  return ($addr & $mask) == ($ip & $mask);  
}