解決に至った回答には300ポイント付与させて頂きます。

こんな感じでどうでしょう？

<?php

define("_CARRIER_PC", "1");
define("_CARRIER_DOCOMO", "2");
define("_CARRIER_SOFTBANK", "3");
define("_CARRIER_EZWEB", "4");

function getCarrierByIP($ip=null) {
  $ranges = array(
    _CARRIER_DOCOMO => array(
      '124.146.174.0/24',
      '210.136.161.0/24',
      '202.229.177.0/24',
    ),
    _CARRIER_SOFTBANK => array(
      '202.179.204.0/24',
      '210.169.130.112/28',
      '211.8.159.128/25',
    ),
    _CARRIER_EZWEB => array(
      '111.86.141.128/26',
      '210.230.128.224/28',
      '219.125.148.160/27',
    ),
  );
  if($ip == null) {
    $ip = $_SERVER['REMOTE_ADDR'];
  }
  foreach($ranges as $carrier => $ranges) {
    foreach($ranges as $range) {
      if(cidr_match($ip, $range)) {
        return $carrier;
      }
    }
  }
  return _CARRIER_PC;
}

function cidr_match($ip, $range) {
  list ($subnet, $bits) = split('/', $range);
  $ip = ip2long($ip);
  $subnet = ip2long($subnet);
  $mask = -1 << (32 - $bits);
  $subnet &= $mask; # nb: in case the supplied subnet wasn't correctly aligned
    return ($ip & $mask) == $subnet;
}

switch(getCarrierByIP()) {
  case _CARRIER_PC:
    //略
    break;
  case _CARRIER_DOCOMO:
    //略
    break;
  case _CARRIER_SOFTBANK:
    //略
    break;
  case _CARRIER_EZWEB:
    //略
    break;
}