strtoupper can be used to update a string to have all upper case characters. For example, let's say you have a variable that contains the text Hello World.

$string = "Hello World";

In this example, the $string will be updated to upper.

$string = strtolower($string);

Now, when you echo $string . . .

echo $string;

. . . the following will be displayed.

HELLO WORLD