    public function __construct($preserve = false) {
        // unreserved letters, ought to const-ify
        for ($i = 48; $i <= 57;  $i++) $this->preserve[$i] = true; // digits
        for ($i = 65; $i <= 90;  $i++) $this->preserve[$i] = true; // upper-case
        for ($i = 97; $i <= 122; $i++) $this->preserve[$i] = true; // lower-case
        $this->preserve[45] = true; // Dash         -
        $this->preserve[46] = true; // Period       .
        $this->preserve[95] = true; // Underscore   _
        $this->preserve[126]= true; // Tilde        ~

        // extra letters not to escape
        if ($preserve !== false) {
            for ($i = 0, $c = strlen($preserve); $i < $c; $i++) {
                $this->preserve[ord($preserve[$i])] = true;
            }
        }
    }
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.