title: php validate if string is alphabetic including cyrillic, greek or any unicode letter
body: <p>I am trying to validate string is alphabetic including multiple character sets:</p>
  
  <pre><code>function is_string($str){
      return preg_match("/^[a-zA-Z\p{Cyrillic}\p{Cyrillic}]+$/u", $str) ? TRUE : FALSE;
  }
  </code></pre>
  
  <p>but it fails if string contains some other characters of different languages (ç, ë are used in albanian language)</p>
  
  <pre><code>is_string('ç');//false
  is_string('ë');//false
  </code></pre>
  
  <p>Is there any general function or something which will fix this problem for any character set?</p>