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Representing IPv6 addresses with PHP per RFC 5952

I wrote the following function in PHP to represent IPv6 addresses as short as possible:

function ipv6_compress($ip){
    // Shorten first group of zeros
    if(substr($ip, 0, 4) == 0000) $ip = substr_replace($ip, '0', 0, 4);
    // Shorten full groups of zeros
    $ip = str_replace('0000:', '0:', $ip);
    // Remove leading zeros
    $ip = preg_replace('/:0{1,3}(?=\w)/', ':', $ip);
    // Remove longest extra group of zeros per [RFC 5952](http://tools.ietf.org/html/rfc5952)
    if(strpos('::') !== false) return $ip; // But don't if a :: is already present as entered
    $pos = strpos($ip, '0:0:0:0:0:0:0:');
    if($pos !== false) return $ip = substr_replace($ip, '::', $pos, 14);
    $pos = strpos($ip, '0:0:0:0:0:0:');
    if($pos !== false) return $ip = substr_replace($ip, '::', $pos, 12);
    $pos = strpos($ip, '0:0:0:0:0:');
    if($pos !== false) return $ip = substr_replace($ip, '::', $pos, 10);
    $pos = strpos($ip, '0:0:0:0:');
    if($pos !== false) return $ip = substr_replace($ip, '::', $pos, 8);
    $pos = strpos($ip, '0:0:0:');
    if($pos !== false) return substr_replace($ip, '::', $pos, 6);
    $pos = strpos($ip, '0:0:');
    if($pos !== false) return substr_replace($ip, '::', $pos, 4);
    return $ip;
}

I attempt to adhere to RFC 5952.

  1. Do I violate the standard in any way?
  2. Can I do anything more efficiently?

As you can probably tell, I am not concerned with validating the addresses.

Mooseman
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