PHP Tips: Filtering ASCII Control Characters_PHP Tutorial

/**
* Filter control characters according to ascii code
* @param type $string
*/
public static function special_filter($string )
{
if(!$string) return '';

$new_string = '';
for($i =0; isset($string[$i]); $i++)
{
$asc_code = ord($string[$i]); //Get its asc code

//The following code is designed to filter illegal characters
if($ asc_code == 9 || $asc_code == 10 || $asc_code == 13){
$new_string .= ' ';
}
else if($asc_code > 31 && $asc_code != 127){
$new_string .= $string[$i];
}
}

return trim($new_string);
}