[Geee]

The null coalescing operator ?? is not well explained. It returns the first operand but if it's not set or null, will return the second operand, i.e:

    $name = $_GET['name'] ?? $_POST['name'] ?? "unknown";

In the arrays section, I'd also introduce the $array[] shorthand for array_push(), i.e:

    $array[] = $new_element;