Binding closures to objects in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to bind the closure to the object and call it.

PHP

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

$person = new Person();

$greet = function() {
    return "Hello, " . $this->name;
};

$boundGreet = $greet->[1]($person);
echo $boundGreet();
?>

Drag options to blanks, or click blank then click option'

Acall

BbindTo

Capply

Dattach

Attempts:

3 left

💡 Hint

Common Mistakes

Using call() instead of bindTo() causes errors because call() immediately invokes the closure.

Using apply() or attach() which do not exist in PHP for closures.

The bindTo method binds a closure to an object, allowing it to use the object's properties via $this.

2fill in blank

medium

Complete the code to bind the closure to the object and specify the class scope.

PHP

<?php
class User {
    private $role = 'admin';
    public function getRoleClosure() {
        return function() {
            return $this->role;
        }->[1]($this, __CLASS__);
    }
}

$user = new User();
$closure = $user->getRoleClosure();
echo $closure();
?>

Drag options to blanks, or click blank then click option'

AbindTo

Bcall

Capply

Dattach

Attempts:

3 left

💡 Hint

Common Mistakes

Not passing the class scope causes private properties to be inaccessible.

Using call() which immediately invokes the closure instead of binding.

bindTo can take a second argument to specify the class scope, allowing access to private properties.

3fill in blank

hard

Fix the error by completing the code to bind the closure to the object correctly.

PHP

<?php
class Product {
    public $price = 100;
}

$product = new Product();

$discount = function($amount) {
    return $this->price - $amount;
};

$boundDiscount = $discount->[1]($product);
echo $boundDiscount(20);
?>

Drag options to blanks, or click blank then click option'

Aapply

Bcall

Cattach

DbindTo

Attempts:

3 left

💡 Hint

Common Mistakes

Using call() which immediately invokes the closure and does not return a bound closure.

Using non-existent methods like apply() or attach().

bindTo binds the closure to the object so $this refers to the object inside the closure.

4fill in blank

hard

Fill both blanks to create a closure bound to the object and call it with an argument.

PHP

<?php
class Calculator {
    public $factor = 2;
}

$calc = new Calculator();

$multiply = function($num) {
    return $num * $this->factor;
};

$boundMultiply = $multiply->[1]([2]);
echo $boundMultiply(5);
?>

Drag options to blanks, or click blank then click option'

AbindTo

Bcall

C$calc

D$multiply

Attempts:

3 left

💡 Hint

Common Mistakes

Using call instead of bindTo returns the result immediately, not a bound closure.

Passing the closure itself instead of the object to bind.

bindTo binds the closure to the object $calc so $this inside the closure refers to $calc.

5fill in blank

hard

Fill all three blanks to bind the closure to the object with class scope and call it with an argument.

PHP

<?php
class Logger {
    private $prefix = '[LOG]';
}

$logger = new Logger();

$logMessage = function($msg) {
    return $this->prefix . ' ' . $msg;
};

$boundLog = $logMessage->[1]([2], [3]);
echo $boundLog('System started');
?>

Drag options to blanks, or click blank then click option'

AbindTo

B$logger

C__CLASS__

Dcall

Attempts:

3 left

💡 Hint

Common Mistakes

Using call instead of bindTo causes immediate invocation, not binding.

Not passing the class scope causes private properties to be inaccessible.

bindTo binds the closure to the object $logger with the class scope __CLASS__ to access private properties.

Practice

1. What does binding a closure to an object in PHP allow the closure to do?

easy

A. Change the object's class permanently

B. Access the object's properties and methods as if it were part of the object

C. Convert the closure into a regular function

D. Prevent the closure from accessing any object data

Solution

1. Step 1: Understand closure binding concept

   Binding a closure to an object lets the closure use the object's properties and methods.

2. Step 2: Identify what binding does

   It does not change the object's class or convert the closure; it only changes the closure's context.

3. Final Answer:

   Access the object's properties and methods as if it were part of the object -> Option B

4. Quick Check:

   Binding closure = access object data [OK]

Hint: Binding closures lets them use object data like methods [OK]

Common Mistakes:

• Thinking binding changes the object's class
• Believing closure becomes a normal function
• Assuming binding blocks access to object data

2. Which of the following is the correct syntax to bind a closure $func to an object $obj in PHP?

easy

A. bindTo($func, $obj);

B. $func->bind($obj);

C. $func->bindTo($obj);

D. $obj->bindClosure($func);

Solution

1. Step 1: Recall the method name for binding closures

   The method to bind a closure to an object is bindTo called on the closure.

2. Step 2: Check syntax correctness

   The correct syntax is $func->bindTo($obj); where $func is the closure and $obj is the object.

3. Final Answer:

   $func->bindTo($obj); -> Option C

4. Quick Check:

   bindTo method on closure = correct syntax [OK]

Hint: Use bindTo() method on closure to bind object [OK]

Common Mistakes:

• Using bind() instead of bindTo()
• Calling bindTo as a global function
• Trying to bind from the object instead of closure

3. What will be the output of the following PHP code?

$obj = new class { private $x = 10; };
$func = function() { return $this->x; };
$bound = $func->bindTo($obj, get_class($obj));
echo $bound();

medium

A. Error: Cannot access private property

B. Closure object

C. null

D. 10

Solution

1. Step 1: Understand closure binding with private scope

   The closure is bound to $obj with the class scope of $obj, allowing access to private properties.

2. Step 2: Evaluate the closure call

   Calling $bound() returns $this->x which is 10, accessible due to binding.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Binding with class scope allows private access = 10 [OK]

Hint: Binding with class scope grants private property access [OK]

Common Mistakes:

• Ignoring class scope in bindTo causing access error
• Expecting null because property is private
• Thinking closure returns itself

4. Identify the error in this PHP code snippet:

$obj = new stdClass();
$func = function() { return $this->value; };
$bound = $func->bindTo($obj);
echo $bound();

medium

A. No error, outputs null

B. Fatal error: Cannot access property 'value' on stdClass

C. Error: Closure not bound to any object

D. Syntax error in closure definition

Solution

1. Step 1: Check if binding is correct

   The closure is bound to $obj which is an instance of stdClass.

2. Step 2: Check property existence and output

   $obj has no value property, so accessing it returns null without error.

3. Final Answer:

   No error, outputs null -> Option A

4. Quick Check:

   Accessing undefined property on stdClass = null [OK]

Hint: Undefined properties on stdClass return null, no error [OK]

Common Mistakes:

• Expecting fatal error for missing property
• Thinking closure is unbound
• Assuming syntax error in closure

5. Given the class below, how can you use a closure bound to an instance to safely increment the private counter property?

class Counter {
  private int $counter = 0;
  public function get() { return $this->counter; }
}

Choose the correct closure binding and usage.

hard

A. $inc = function() { $this->counter++; }; $inc = $inc->bindTo($obj, Counter::class); $inc();

B. $inc = function() { $this->counter++; }; $inc();

C. $inc = function() { $this->counter++; }; $inc = $inc->bindTo($obj); $inc();

D. $inc = function() { $this->counter++; }; $inc = $inc->bindTo(null, Counter::class); $inc();

Solution

1. Step 1: Understand private property access

   To access private counter, closure must be bound to object $obj with class scope Counter::class.

2. Step 2: Check binding and invocation

   $inc = function() { $this->counter++; }; $inc = $inc->bindTo($obj, Counter::class); $inc(); binds closure to $obj and class Counter, then calls it to increment counter.

3. Final Answer:

   $inc = function() { $this->counter++; }; $inc = $inc->bindTo($obj, Counter::class); $inc(); -> Option A

4. Quick Check:

   Bind closure to object and class for private access [OK]

Hint: Bind closure with object and class to access private properties [OK]

Common Mistakes:

• Not specifying class scope in bindTo
• Calling closure without binding
• Binding with null object or missing object