Matching IPv6 address to a CIDR subnet

I created my own solution, using the following code:

function iPv6MaskToByteArray($subnetMask) {
  $addr = str_repeat("f", $subnetMask / 4);
  switch ($subnetMask % 4) {
    case 0:
      break;
    case 1:
      $addr .= "8";
      break;
    case 2:
      $addr .= "c";
      break;
    case 3:
      $addr .= "e";
      break;
  }
  $addr = str_pad($addr, 32, '0');
  $addr = pack("H*" , $addr);
  return $addr;
}

function iPv6CidrMatch($address, $subnetAddress, $subnetMask) {
  $binMask = iPv6MaskToByteArray($subnetMask);
  return ($address & $binMask) == $subnetAddress;
}

Note that $address and $subnetAddress were obtained by running the string address through inet_pton. Call the function as follows:

$subnet = inet_pton("2001:06b8::");
$mask = 32;
$addr = inet_pton("2001:06b8:0000:0000:0000:0000:1428:07ab");
$match = iPv6CidrMatch($addr, $subnet, $mask); // TRUE

You can also use the IpUtils class from symfony/http-foundation package:

IpUtils::checkIp6('2a01:8760:2:3001::1', '2a01:8760:2:3001::1/64')

This will check the IPv6 validity and range match. Will return false if it's not the case.

Since you cannot convert IPv6 addresses to integer, you should operate bits, like this:

$ip='21DA:00D3:0000:2F3B:02AC:00FF:FE28:9C5A';
$cidrnet='21DA:00D3:0000:2F3B::/64';

// converts inet_pton output to string with bits
function inet_to_bits($inet) 
{
   $splitted = str_split($inet);
   $binaryip = '';
   foreach ($splitted as $char) {
             $binaryip .= str_pad(decbin(ord($char)), 8, '0', STR_PAD_LEFT);
   }
   return $binaryip;
}    

$ip = inet_pton($ip);
$binaryip=inet_to_bits($ip);

list($net,$maskbits)=explode('/',$cidrnet);
$net=inet_pton($net);
$binarynet=inet_to_bits($net);

$ip_net_bits=substr($binaryip,0,$maskbits);
$net_bits   =substr($binarynet,0,$maskbits);

if($ip_net_bits!==$net_bits) echo 'Not in subnet';
else echo 'In subnet';

Also, if you use some database to store IPs, it may already have all the functions to compare them. For example, Postgres has an inet type and can determine, whether IP is contained within subnet like this:

SELECT 
   '21DA:00D3:0000:2F3B:02AC:00FF:FE28:9C5A'::inet << 
   '21DA:00D3:0000:2F3B::/64'::inet;

9.11. Network Address Functions and Operators in PostgreSQL