Passing arguments to functions and returning values Mocktest

Callbacks in PHP allow passing a function as an argument to another function, providing flexibility and enabling advanced programming techniques such as event handling and functional programming paradigms.

The output of the code will be HELLO because the uppercase() function accepts a string $str as a parameter and returns its uppercase version using the strtoupper() function. The function is then called with the argument "hello" and its result is echoed.