Simple algorithm to quickly obfuscate a string in javascript.

obfuscate.js

/**
 * Obfuscate a plaintext string with a simple rotation algorithm similar to
 * the rot13 cipher.
 * @param  {[type]} key rotation index between 0 and n
 * @param  {Number} n   maximum char that will be affected by the algorithm
 * @return {[type]}     obfuscated string
 */
String.prototype.obfs = function(key, n = 126) {
  // return String itself if the given parameters are invalid
  if (!(typeof(key) === 'number' && key % 1 === 0)
    || !(typeof(key) === 'number' && key % 1 === 0)) {
    return this.toString();
  }

  var chars = this.toString().split('');

  for (var i = 0; i < chars.length; i++) {
    var c = chars[i].charCodeAt(0);

    if (c <= n) {
      chars[i] = String.fromCharCode((chars[i].charCodeAt(0) + key) % n);
    }
  }

  return chars.join('');
};

/**
 * De-obfuscate an obfuscated string with the method above.
 * @param  {[type]} key rotation index between 0 and n
 * @param  {Number} n   same number that was used for obfuscation
 * @return {[type]}     plaintext string
 */
String.prototype.defs = function(key, n = 126) {
  // return String itself if the given parameters are invalid
  if (!(typeof(key) === 'number' && key % 1 === 0)
    || !(typeof(key) === 'number' && key % 1 === 0)) {
    return this.toString();
  }

  return this.toString().obfs(n - key);
};

Equivalent in PHP

function rotn($txt, $shift = 13, $n = 126) {
	$array_text = str_split($txt);
	foreach($array_text as $key => $value){
		if(ord($value) <= $n){
			$new_ascii = (ord($value) + $shift);
			if($new_ascii >= 0){
				$array_text[$key] = chr($new_ascii);
			} else {
				$array_text[$key] = chr($n + $new_ascii);
			}
		}
	}
    return implode('',$array_text);
}
$txt = 'Hello Word';
echo rotn($txt);