title: How to transform a string to lowercase with preg_replace
body: <p>I just stuck at this and cannot find solution. 
  I would like to try to transform a string to lower case using <code>preg_replace</code>.
  I just cannot create the right regex.
  The reason is that normal <code>strtolower</code> does not support unicode characters.
  I know that I could use <code>mb_strtolower</code> but this function seems to be quite slow and beside them not everyone has MB support.</p>
  
  <p>Any clue?</p>
  
  <p>Regards,
  Radek</p>
  
  <p>EDIT: Ok, thanks alot for your help guys. I think my approach was not quite correct.
  I think it would be much better to use this: <a href="https://stackoverflow.com/questions/6497685/how-do-i-detect-non-ascii-characters-in-a-string">How do I detect non-ASCII characters in a string?</a> and then respectively use either the strtolower or mb_strtolower if available.</p>