PHP similar_text algorithm

The similar_text returns the number of matching letters of two strings. It can also calculate the similarity of the two strings in percent (matched letters count/average length of two letters)*100. For example,

<?php
echo similar_text("Hello World","Hello Peter");
?>
Output: 7

<?php
echo similar_text("Pello World","Hello Weter");
?>
Output: 6, because the letters are not in the correct order.

<?php
similar_text("Hello World","Hello Peter",$percent);
echo $percent;
?>
Output: 63.6363636364