use($app)

Hey! This is more of a scoping-ee thing than anything else.

consider this:

<?php

$veryImportantName = 'Tom';

$importantPeople = array_filter($people, function($person) {
    return ($veryImportantName == $person['name'])
});

Now, array filter function accepts a callback as the second argument. You might've picked up scoping in previous videos. Essentially you can't use thing defined OUTSIDE of the function INSIDE the function. The above example would fail because inside that callback, $veryImportantName isn't accessible.

What you're doing is saying to the callback "HEY! There's this really cool variable I want to use inside you.".

It's handy because in a callback you can't add additional arguments. To make the above work, you'd need:

<?php

$veryImportantName = 'Tom';

$importantPeople = array_filter($people, function($person) use($veryImportantName) {
    return ($veryImportantName == $person['name'])
});

Check out the docs on anonymous functions

Hope this helps!