NOTE: here in the S.O. there are many ways to do this with array keys, between values/words of the same variables, between two different arrays, but I see nothing on the internet between strings and not arrays of two different variables.
<?php
$a = 'this car is very beautiful and is the fast';
$b = 'this red car is very beautiful and is the fast that others';
var_export($unique_words = show_unique_strings($a, $b));
//expected output(painted on the screen): red that others
?>