Hey,

So I need to be the mean part here, I'm really sorry :/.

But there are a few "wrongs" in this article and I need to clarify

Syntax

While you CAN write ?> at the end of your files, you really SHOULDN'T. In Javascript there are multiple competing syntax-standards (like the one from AirBNB or the "Standard" standard.), but PHP is more like Python: There are PSR-2 and PSR-12 which describe exactly how standard-compliant code should look like. No more fights over indentation and so on ;).
php-fig.org/psr/psr-12/

Rules 2.2.:

The closing ?> tag MUST be omitted from files containing only PHP.

Emphasis on the "only".

Standards

Btw the next thing missing (maybe you didn't stumble upon it yet) are other references to the PSR. To step up your game, you really should try to understand all of them. But PLEASE refer from PSR-8, as it's the wrong time for now.

Declaring Variables

define doesn't really declare Variables, but compile time constants. They really can't be changed by any means and are like a little chipmunk replacing all the occurences with it's true value. This is the reason why you can't write sth. like:

define(POWERLEVEL, $sayajin->determinePowerlevel());

What you CAN do is simple math:

define(POWERLEVEL, 2*9*500);

Declaring Arrays

Not really about declaring arrays, but you say, that an Array is the only way to create a collection of key/value pairs without importing.
But you really can (and should) use data classes which hold your data and data collection. Like in javascript you can use them in parameters without their content to be copied by whole, but as a reference!

Functions

Nothing wrong here, but keep in mind, that all Objects are implicitly called by reference, while all scalars (int, string, arrays) are copied by value!

function applySomeProperty($someClass) {
    $someClass->bestBuddy = "Kuririn";
}

$unimportantClass = new class {};
applySomeProperty($unimportantClass);
echo $unimportantClass->newProperty; // Kuririn

And while we're here: Since the 7.x Versions PHP supports types!

function someFunction(int $age, string $name): PersonObject {}

You really should use them!

Classes

Not much here, but it's important to know, that you use :: in static context, while -> in private. Additionally there's a difference between $this::$varname and self::$varname.