Methods and $this keyword in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method using $this keyword

What is the output of this PHP code?

PHP

<?php
class Car {
    public $color = 'red';
    public function getColor() {
        return $this->color;
    }
}
$car = new Car();
echo $car->getColor();
?>

A Error: Undefined property

B Car

C null

D red

💡 Hint

Remember $this refers to the current object instance.

The method getColor() returns the $color property of the current object using $this->color. Since $color is set to 'red', the output is 'red'.

❓ Predict Output

intermediate

Effect of missing $this in method

What error or output does this PHP code produce?

PHP

<?php
class Person {
    public $name = 'Alice';
    public function sayName() {
        return name;
    }
}
$p = new Person();
echo $p->sayName();
?>

A Error: Undefined constant 'name'

B Alice

C null

D Empty string

💡 Hint

Check how properties are accessed inside methods.

Inside the method, name without $this-> is treated as a constant. Since no constant named 'name' exists, PHP throws an error.

🔧 Debug

advanced

Why does this method not update the property?

Consider this PHP code. Why does the setAge method not change the $age property of the object?

PHP

<?php
class User {
    public $age = 20;
    public function setAge($age) {
        $age = $age;
    }
}
$user = new User();
$user->setAge(30);
echo $user->age;
?>

A Because the method is static and cannot access instance properties.

B Because $age property is private and cannot be changed.

C Because the method assigns the parameter to itself, not to the property using $this.

D Because the property $age is not declared in the class.

💡 Hint

Look at how the assignment is done inside the method.

The method assigns the parameter $age to itself, which does nothing. To update the object's property, it should use $this->age = $age;.

📝 Syntax

advanced

Identify the syntax error in method using $this

Which option contains the correct syntax to return the $name property inside a method?

PHP

<?php
class Animal {
    public $name = 'Dog';
    public function getName() {
        // return statement here
    }
}
?>

A return $this->name;

B return this->$name;

C return $this.name;

D return $name;

💡 Hint

Remember the correct syntax to access properties with $this.

To access an object property inside a method, use $this->propertyName. Other options have syntax errors or incorrect variable usage.

🚀 Application

expert

Predict the output of chained method calls using $this

What is the output of this PHP code?

PHP

<?php
class Builder {
    private $text = '';
    public function addHello() {
        $this->text .= 'Hello ';
        return $this;
    }
    public function addWorld() {
        $this->text .= 'World!';
        return $this;
    }
    public function getText() {
        return $this->text;
    }
}
$b = new Builder();
echo $b->addHello()->addWorld()->getText();
?>

A Error: Cannot chain methods

B Hello World!

C World!

D Hello

💡 Hint

Check how methods return $this to allow chaining.

Each method appends text and returns $this, allowing chaining. The final getText() returns the full string 'Hello World!'.

Practice

1. What does the $this keyword represent inside a PHP class method?

easy

A. A global variable

B. A static variable

C. The current object instance

D. The class name

Solution

1. Step 1: Understand the role of $this in PHP classes

   $this is used inside class methods to refer to the current object instance that called the method.

2. Step 2: Differentiate from other options

   It is not a static or global variable, nor does it represent the class name itself.

3. Final Answer:

   The current object instance -> Option C

4. Quick Check:

   $this = current object instance [OK]

Hint: Remember: $this always means 'this object' [OK]

Common Mistakes:

• Confusing $this with static variables
• Thinking $this is the class name
• Assuming $this is a global variable

2. Which of the following is the correct way to call a method named greet inside a PHP class using $this?

easy

A. this->greet();

B. self->greet();

C. $this.greet();

D. $this->greet();

Solution

1. Step 1: Recall PHP method call syntax inside classes

   Methods are called on objects using the arrow operator -> with $this representing the current object.

2. Step 2: Check each option

   $this->greet(); is the correct syntax. The other options use invalid syntax.

3. Final Answer:

   $this->greet(); -> Option D

4. Quick Check:

   Method call syntax = $this->method(); [OK]

Hint: Use $this->methodName(); to call methods inside class [OK]

Common Mistakes:

• Using dot (.) instead of arrow (->) operator
• Omitting the dollar sign in $this
• Using self without scope resolution operator

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

class Person {
  public $name = 'Alice';
  public function sayName() {
    return 'Name: ' . $this->name;
  }
}
$p = new Person();
echo $p->sayName();

medium

A. Name: Alice

B. Name: $name

C. Name:

D. Error: Undefined property

Solution

1. Step 1: Understand property access with $this

   The method sayName() returns the string 'Name: ' concatenated with the object's name property accessed via $this->name.

2. Step 2: Check the property value and output

   The property name is set to 'Alice', so the output will be 'Name: Alice'.

3. Final Answer:

   Name: Alice -> Option A

4. Quick Check:

   Property accessed via $this = 'Alice' [OK]

Hint: Remember: $this->property accesses current object's data [OK]

Common Mistakes:

• Forgetting to use $this to access properties
• Expecting literal string output without variable substitution
• Confusing property name with string '$name'

4. Identify the error in this PHP class method:

class Car {
  public $model = 'Tesla';
  public function getModel() {
    return $model;
  }
}

medium

A. Missing $this when accessing $model

B. Method name should be GetModel with capital G

C. Property $model must be private

D. Return statement should be return $this->getModel;

Solution

1. Step 1: Check property access inside method

   Inside class methods, properties must be accessed with $this->propertyName. Here, $model is used without $this->.

2. Step 2: Validate other options

   Method names are case-insensitive in PHP, property visibility is not an error here, and the suggested return $this->getModel; is incorrect.

3. Final Answer:

   Missing $this when accessing $model -> Option A

4. Quick Check:

   Use $this->property to access properties [OK]

Hint: Always use $this-> to access object properties inside methods [OK]

Common Mistakes:

• Accessing properties without $this->
• Confusing method name case sensitivity
• Misunderstanding property visibility rules

5. Given the class below, what will be the output?

class Counter {
  private $count = 0;
  public function increment() {
    $this->count++;
  }
  public function getCount() {
    return $this->count;
  }
}
$c = new Counter();
$c->increment();
$c->increment();
echo $c->getCount();

hard

A. Error: Cannot access private property

B. 2

C. 1

D. 0

Solution

1. Step 1: Understand property increment inside the class

   The increment() method increases the private $count property by 1 each time it is called.

2. Step 2: Trace method calls and final output

   Calling increment() twice increases $count from 0 to 2. Then getCount() returns 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Two increments = count 2 [OK]

Hint: Private properties accessed via $this inside class work fine [OK]

Common Mistakes:

• Assuming private properties can't be accessed inside class methods
• Forgetting that increment operator changes value
• Expecting output before increments