code: public static function q($string)
      {
          # HACK: "self::SPECIAL_ESCAPE_OUT[...]" is not valid syntax in PHP 5.3, so put
          # it in a local variable first.
          $special_escape_out = self::SPECIAL_ESCAPE_OUT;
  
          $r = "\"";
          $len = \strlen($string);
          for ($i = 0; $i < $len; $i++) {
              $c = $string[$i];
              $escape_i = \strpos(self::SPECIAL_ESCAPE_IN, $c);
              if ($escape_i !== false) {
                  // Characters with a special escape code.
                  $r .= "\\";
                  $r .= $special_escape_out[$escape_i];
              }
              else if ($c >= "\x20" and $c <= "\x7e") {
                  // Printable characters.
                  $r .= $c;
              }
              else {
                  // Generic hex escape code.
                  $r .= "\\x";
                  $r .= \bin2hex($c);
              }
          }
          $r .= "\"";
          return $r;
      }
docs: Return a double-quoted version of the given string, using PHP-escape sequences
  for all non-printable and non-ASCII characters.
  
  @param string $string
  
  @return string