Here are my versions of prefix to mask and mask to prefix conversions. You can remove first lines with if/preg of each function if you don't want sanity precheck.
function prefix2Mask($prefix)
{
if (!preg_match('#^(?:0*[0-2]?[0-9]|3[0-2])$#', $prefix)) return false; # wrong prefix
return long2ip(0xffffffff << (32 - $prefix));
}
function mask2Prefix($mask)
{
if (!preg_match('#^(?:(?:[0-1][0-9][0-9]|2[0-4][0-9]|25[0-5]|[0-9][0-9]|[0-9])\.){3}'.
'(?:[0-1][0-9][0-9]|2[0-4][0-9]|25[0-5]|[0-9][0-9]|[0-9])$#', $mask))
return false; # wrong mask
$m2p = str_pad(substr(decbin(ip2long($mask)), -32, 32), 32, '0', STR_PAD_LEFT);
if (!preg_match('#^(1*)0*$#', $m2p, $m)) return false; # wrong mask
return strlen($m[1]);
}
