PHP Class Methods: Add Behaviour to Your Objects

A method is a function that belongs to a class. It describes something an object can do.

index.php

PHP

<?php
class Rectangle
{
    public function __construct(
        public float $width,
        public float $height,
    ) {}

    public function area(): float
    {
        return $this->width * $this->height;
    }

    public function isSquare(): bool
    {
        return $this->width === $this->height;
    }
}

$r = new Rectangle(4, 5);

echo $r->area() . "\n";
var_dump($r->isSquare());

Output

OUTPUT

1
2

20
bool(false)

Methods are written like functions, without the function word being different: public function area(): float. They can have parameters, return types and default values, exactly as in the functions chapter.

Methods use the object’s data

Inside a method, $this is the current object. $this->width reads that object’s own width. See $this.

Methods that change the object

index.php

PHP

<?php
class Counter
{
    private int $count = 0;

    public function increment(): void
    {
        $this->count++;
    }

    public function value(): int
    {
        return $this->count;
    }
}

$c = new Counter();
$c->increment();
$c->increment();

echo $c->value();

Output

OUTPUT

Method chaining

If a method returns $this, you can call another method on the result, in one line. This is called a fluent interface.

index.php

PHP

<?php
class Query
{
    private array $parts = [];

    public function select(string $columns): static
    {
        $this->parts[] = "SELECT $columns";
        return $this;
    }

    public function from(string $table): static
    {
        $this->parts[] = "FROM $table";
        return $this;
    }

    public function build(): string
    {
        return implode(" ", $this->parts);
    }
}

echo (new Query())->select("name, price")->from("products")->build();

Output

OUTPUT

SELECT name, price FROM products

Method names

Use camelCase and start with a verb: getTotal(), addItem(), isPaid(), hasStock(). Methods that return a boolean usually start with is, has or can.

Keep methods small

A method should do one thing and fit on one screen. If a method has an “and” in its description (“it validates and saves and sends an email”), split it.

Calling a method that does not exist

index.php

PHP

<?php
class Dog {}

try {
    (new Dog())->fly();
} catch (Error $e) {
    echo $e->getMessage();
}

Output

OUTPUT

Call to undefined method Dog::fly()

Methods can call each other

index.php

PHP

<?php
class Cart
{
    private array $prices = [100, 250];

    public function subtotal(): float
    {
        return array_sum($this->prices);
    }

    public function total(): float
    {
        return $this->subtotal() * 1.18;
    }
}

echo (new Cart())->total();

Output

OUTPUT

Exercise: a bank account

Create a class Account with a private balance. Add deposit(float $amount), withdraw(float $amount) (do not allow going below 0) and balance(): float.

Show a possible solution

index.php

PHP

<?php
class Account
{
    private float $balance = 0;

    public function deposit(float $amount): void
    {
        $this->balance += $amount;
    }

    public function withdraw(float $amount): bool
    {
        if ($amount > $this->balance) {
            return false;
        }

        $this->balance -= $amount;
        return true;
    }

    public function balance(): float
    {
        return $this->balance;
    }
}

$account = new Account();
$account->deposit(500);
var_dump($account->withdraw(800));
$account->withdraw(200);

echo $account->balance();

Output

OUTPUT

1
2

bool(false)
300