- strtolower: documentation ( source)
<?php
$string = "AS A SINGLE FOOTSTEP WILL NOT MAKE A PATH ON THE EARTH, SO A SINGLE THOUGHT WILL NOT MAKE A PATHWAY IN THE MIND. TO MAKE A DEEP PHYSICAL PATH, WE WALK AGAIN AND AGAIN. TO MAKE A DEEP MENTAL PATH, WE MUST THINK OVER AND OVER, THE KIND OF THOUGHTS WE WISH TO DOMINATE OUR LIVES.";
echo strtolower($string);