What's the difference between using normal functions and class methods in PHP?

Question

I'm trying to learn how to best use OOP in PHP. Please be aware that even if I studied the theory of this new "world" I didn't enter the OOP thinking yet obviously.

What's the difference between using normal, separated functions and putting them in a class as methods?

Let's say I have a class called "shop".

It has these methods: retrieveitems, deleteitems, updateitems, additems

Except for the fact that I can call methods inside methods with a simple "$this", what is the difference between putting them in different functions without a class? I mean, for example, I still can call function deleteitems inside function retrieveitems right? Even if not in a class?

Please help me understand what I'm missing.

Answer 1

OOP provides, among other things, encapsulation.

class Shop {
    
    function __construct($items) {
        $this->inventory = $items;
    }

    function deleteItem($item) {
        $key = array_search($item, $this->inventory);
        if ($key !== false)
            unset($this->inventory[$key]);
    }

}

Now you can create instances:

$computerShop  = new Shop(array('ram', 'monitor', 'cpu', 'water'));
$hardwareStore = new Shop(array('hammer', 'screwdriver', 'water'));

And each one of them is independent from each other. If I do $computerShop->removeItem('water'), $hardwareStore should still have water in their inventory. (see it in action) Doing this the procedural way is much messier.

---

Another cool thing about OOP is that you can use inheritance and polymorphism:

class Animal {

    function eat() {
        $this->hungry = false;
    }

    abstract function speak();
}

class Cat extends Animal {

    function speak() {
        echo 'meow!';
    }
}

class Dog extends Animal {
    
    function speak() {
        echo 'woof!';
    }    
}

Now both Cat and Dogs can call the method eat() even though they are not explicitly declared in their classes - it's been inherited from their parent class Animal. They also have a speak() method that does different things. Pretty neat, huh?

Wikipedia:

Object-oriented programming, Encapsulation, Inheritance, Polymorphism