function htmlspecialchars (string, quote_style, charset, double_encode) {
  // http://kevin.vanzonneveld.net
  // +   original by: Mirek Slugen
  // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   bugfixed by: Nathan
  // +   bugfixed by: Arno
  // +    revised by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +    bugfixed by: Brett Zamir (http://brett-zamir.me)
  // +      input by: Ratheous
  // +      input by: Mailfaker (http://www.weedem.fr/)
  // +      reimplemented by: Brett Zamir (http://brett-zamir.me)
  // +      input by: felix
  // +    bugfixed by: Brett Zamir (http://brett-zamir.me)
  // %        note 1: charset argument not supported
  // *     example 1: htmlspecialchars("<a href='test'>Test</a>", 'ENT_QUOTES');
  // *     returns 1: '&lt;a href=&#039;test&#039;&gt;Test&lt;/a&gt;'
  // *     example 2: htmlspecialchars("ab\"c'd", ['ENT_NOQUOTES', 'ENT_QUOTES']);
  // *     returns 2: 'ab"c&#039;d'
  // *     example 3: htmlspecialchars("my "&entity;" is still here", null, null, false);
  // *     returns 3: 'my &quot;&entity;&quot; is still here'
  var optTemp = 0,
    i = 0,
    noquotes = false;
  if (typeof quote_style === 'undefined' || quote_style === null) {
    quote_style = 2;
  }
  string = string.toString();
  if (double_encode !== false) { // Put this first to avoid double-encoding
    string = string.replace(/&/g, '&amp;');
  }
  string = string.replace(/</g, '&lt;').replace(/>/g, '&gt;');

  var OPTS = {
    'ENT_NOQUOTES': 0,
    'ENT_HTML_QUOTE_SINGLE': 1,
    'ENT_HTML_QUOTE_DOUBLE': 2,
    'ENT_COMPAT': 2,
    'ENT_QUOTES': 3,
    'ENT_IGNORE': 4
  };
  if (quote_style === 0) {
    noquotes = true;
  }
  if (typeof quote_style !== 'number') { // Allow for a single string or an array of string flags
    quote_style = [].concat(quote_style);
    for (i = 0; i < quote_style.length; i++) {
      // Resolve string input to bitwise e.g. 'ENT_IGNORE' becomes 4
      if (OPTS[quote_style[i]] === 0) {
        noquotes = true;
      }
      else if (OPTS[quote_style[i]]) {
        optTemp = optTemp | OPTS[quote_style[i]];
      }
    }
    quote_style = optTemp;
  }
  if (quote_style & OPTS.ENT_HTML_QUOTE_SINGLE) {
    string = string.replace(/'/g, '&#039;');
  }
  if (!noquotes) {
    string = string.replace(/"/g, '&quot;');
  }

  return string;
}

htmlspecialchars("<a href='test'>Test</a>", 'ENT_QUOTES');

'<a href='test'>Test</a>'

htmlspecialchars("ab\"c'd", ['ENT_NOQUOTES', 'ENT_QUOTES']);

'ab"c'd'

htmlspecialchars("my "&entity;" is still here", null, null, false);

'my "&entity;" is still here'