IP Range 2 CIDR

function range2cidr($ip_start, $ip_end) {

    static $cidr2hosts;

    if(empty($cidr2hosts)){

        $cidr2hosts = array(
            32 => 1,
            31 => 2,
            30 => 4,
            29 => 8,
            28 => 16,
            27 => 32,
            26 => 64,
            25 => 128,
            24 => 256,
            23 => 512,
            22 => 1024,
            21 => 2048,
            20 => 4096,
            19 => 8192,
            18 => 16384,
            17 => 32768,
            16 => 65536,
            15 => 131072,
            14 => 262144,
            13 => 524288,
            12 => 1048576,
            11 => 2097152,
            10 => 4194304,
            9  => 8388608,
            8  => 16777216,
            7  => 33554432,
            6  => 67108864,
            5  => 134217728,
            4  => 268435456,
            3  => 536870912,
            2  => 1073741824,
            1  => 2147483648,
            0  => 4294967296
        );
    }

    $arr_start = explode(".", $ip_start);
    $arr_end = explode(".", $ip_end);

    $bin_start = "";
    $bin_end = "";

    foreach($arr_start as $a){

        $bin_start.= str_pad(decbin($a), 8, "0", STR_PAD_LEFT);
    }

    foreach($arr_end as $a){
        $bin_end.= str_pad(decbin($a), 8, "0", STR_PAD_LEFT);
    }

    $cidr = 0;
    for($x=0;$x<strlen($bin_start);$x++){
        if($bin_start[$x] == $bin_end[$x]){
            $cidr++;
        } else {
            break;
        }
    }

    $host_count = ~(ip2long($ip_start) ^ ip2long($ip_end)) * -1;

    $cidrs = array();

    if($cidr2hosts[$cidr] != $host_count){

        // we have a non-cidr range, break it up and find them all

        $ranges = array();

        $new_end = long2ip(ip2long($ip_end) - ($cidr2hosts[$cidr] - $host_count));

        $ranges[] = array($ip_start, $new_end);

        $new_start = long2ip(ip2long($new_end) + 1);
        $ranges[] = array($new_start, $ip_end);

        foreach($ranges as $range){
            $cidrs = array_merge($cidrs, call_user_func_array("range2cidr", $range));
        }

    } else {

        $cidrs = array("$ip_start/$cidr");

    }

    return $cidrs;
}