Quick way of expanding IPv6 Addresses with PHP

The following is a two liner, where $ip is a condensed IPv6 address. Returns expanded $ip.

Example:

$ip = "fe80:01::af0";
echo expand($ip); // fe80:0001:0000:0000:0000:0000:0000:0af0

Function:

function expand($ip){
    $hex = unpack("H*hex", inet_pton($ip));         
    $ip = substr(preg_replace("/([A-f0-9]{4})/", "$1:", $hex['hex']), 0, -1);

    return $ip;
}

With the help from Mike Mackintosh and Yann Milin I came up with this function:

function expandIPv6($ip) {
    $hex = bin2hex(inet_pton($ip));
    return implode(':', str_split($hex, 4));
}

Below a more universal function witch will also extract IPv4 from an IPv4-mapped IPv6 addresses:

function expandIPv6($ip) {
    if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4))
        return $ip;
    elseif (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
        $hex = bin2hex(inet_pton($ip));
        if (substr($hex, 0, 24) == '00000000000000000000ffff') // IPv4-mapped IPv6 addresses
            return long2ip(hexdec(substr($hex, -8)));
        return implode(':', str_split($hex, 4));
    }
    else
        return false;
}