$this and Object Context

Inside an Instance Method, $this refers to the specific Object on which that Method is currently running.

The current Object

final class Wallet
{
    public function __construct(
        public float $balance,
    ) {}

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

$first = new Wallet(100);
$second = new Wallet(500);

$first->deposit(20);
$second->deposit(50);

During the first call, $this refers to $first; during the second, it refers to $second. The Method has one definition, but it operates on the State of the receiving Object.

Accessing Instance Members

$this->property accesses an Instance Property. $this->method() calls another Instance Method on the same Object:

final class Wallet
{
    public function __construct(
        public float $balance,
    ) {}

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

    private function ensurePositive(float $amount): void
    {
        if ($amount <= 0) {
            throw new InvalidArgumentException();
        }
    }
}

Both accesses use the current Wallet Object.

Static Context

A Static Method belongs to the Class-level context and is called without a receiving Object:

Wallet::showCurrent();

There is no current Object in that call, so $this is unavailable. Trying to use it from a Static Method fails with an error such as:

Using $this when not in object context

Static Members and their trade-offs are covered in depth in a later lesson.

Practical example

final class Counter
{
    public function __construct(
        public int $value,
    ) {}

    public function increment(): void
    {
        $this->value += 1;
    }

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

$counter = new Counter(5);
$counter->incrementTwice();

echo $counter->value; // 7

Common mistakes

• Thinking $this represents every Object created from the Class.
• Trying to use $this in Static Context.
• Calling an Instance Member as if it were a Class-level Member.
• Forgetting that the receiving Object determines which State changes.
• Omitting void from Methods that intentionally return no value.

Summary

$this->property = access State on the current Object.
$this->method() = call Behavior on the current Object.
Static Context  = no current Object, therefore no $this.

Knowledge check

1. What determines the Object referenced by $this?
2. What does $this->increment() mean inside a Method?
3. Why is $this unavailable in a Static Method?