Binding closures to objects in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using closure binding?

Consider the following PHP code. What will it output when run?

PHP

<?php
class Person {
    private string $name = 'Alice';
}

$closure = function() {
    return $this->name;
};

$person = new Person();
$boundClosure = $closure->bindTo($person, Person::class);
echo $boundClosure();
?>

A NULL

B Fatal error: Uncaught Error: Cannot access private property Person::$name

C Alice

D Error: Closure binding failed

💡 Hint

The closure is bound to the Person object with the correct scope.

The closure is bound to the Person object and its class scope, so it can access the private property $name. It returns 'Alice'.

❓ Predict Output

intermediate

What error does this PHP closure binding code produce?

What error will this code produce when executed?

PHP

<?php
class Animal {
    private string $type = 'Cat';
}

$closure = function() {
    return $this->type;
};

$animal = new Animal();
$boundClosure = $closure->bindTo($animal, 'stdClass');
echo $boundClosure();
?>

A Error: Closure binding failed

B Cat

C NULL

D Fatal error: Uncaught Error: Cannot access private property Animal::$type

💡 Hint

The closure is bound with a different class scope than the object's class.

Binding the closure to the Animal object but with 'stdClass' scope means it cannot access Animal's private properties, causing a fatal error.

🔧 Debug

advanced

Why does this closure binding code cause an error?

This PHP code tries to bind a closure to an object but causes an error. What is the cause?

PHP

<?php
class Car {
    private string $model = 'Tesla';
}

$closure = function() {
    return $this->model;
};

$car = new Car();
$boundClosure = $closure->bindTo(null, Car::class);
echo $boundClosure();
?>

A Fatal error because binding to null means no object context for $this

B Outputs 'Tesla' because scope is Car::class

C Outputs NULL because no object is bound

D Syntax error due to missing semicolon

💡 Hint

Binding to null removes the object context, so $this is undefined.

Binding the closure to null removes the object context, so inside the closure $this is not defined, causing a fatal error when accessing $this->model.

❓ Predict Output

advanced

What is the output of this closure bound to a subclass object?

What will this PHP code output?

PHP

<?php
class Vehicle {
    protected string $type = 'Vehicle';
}

class Bike extends Vehicle {
    private string $type = 'Bike';
}

$closure = function() {
    return $this->type;
};

$bike = new Bike();
$boundClosure = $closure->bindTo($bike, Bike::class);
echo $boundClosure();
?>

A Bike

B Vehicle

C Fatal error: Cannot access private property Bike::$type

D NULL

💡 Hint

The closure is bound with Bike class scope, so it accesses Bike's private property.

Binding the closure to the Bike object with Bike class scope allows access to Bike's private $type property, which is 'Bike'.

🧠 Conceptual

expert

How many items are in the resulting array after binding and invoking closures?

Given this PHP code, how many elements does the resulting array contain?

PHP

<?php
class Box {
    private int $count = 3;
}

$closure = function() {
    return [$this->count, $this->count * 2];
};

$box = new Box();
$boundClosure = $closure->bindTo($box, Box::class);
$result = $boundClosure();
print_r($result);
?>

A 1

B 2

C 0

D Fatal error

💡 Hint

The closure returns an array with two elements.

The closure returns an array with two elements: the private property count (3) and its double (6). So the array has 2 elements.