PHP Function To Get The Last N Characters From A String

This is a little PHP function to get the last n characters in a string. As you can see, it's pretty simple to do this with the native substr() function. I just forget how to do it a lot, so wrapping it in a method makes life a little easier for me.

<?php
  /**
  * Returns the last $n chararacters of a $str.
  *
  */
  public function LastNChars($str, $n)
  {
    return substr($str, (-1*$n), -1);
  }
?>