Decited solution was to enable arrays of protocols to be passed to PrependString, and if non match, prepend the first:

function PrependString($Prepend, $String) {
if ($String == '') return '';

if (is_array($Prepend)){
foreach ($Prepend as $str) {
$pos = strpos(strtolower($String), strtolower($str));
if ($pos !== false && $pos == 0) {
$Prepend = '';
}
}
// If the string doesn't start with any array elements, prepend the first element
if ($Prepend <> '') $Prepend = $Prepend[0];
} else {
$pos = strpos(strtolower($String), strtolower($Prepend));
if ($pos !== false && $pos == 0) $Prepend = '';
}

return $Prepend.$String;
}