Instanceof operator in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using instanceof?

Consider the following PHP code snippet. What will it output?

PHP

<?php
class Animal {}
class Dog extends Animal {}
$dog = new Dog();
if ($dog instanceof Animal) {
    echo 'Yes';
} else {
    echo 'No';
}
?>

A No

B Yes

C Fatal error

D null

💡 Hint

Remember that instanceof checks if an object is an instance of a class or its subclass.

The object $dog is an instance of Dog, which extends Animal. So $dog instanceof Animal returns true, printing 'Yes'.

❓ Predict Output

intermediate

What does this code output when checking interface with instanceof?

Given this PHP code, what will be printed?

PHP

<?php
interface Flyer {}
class Bird implements Flyer {}
$bird = new Bird();
if ($bird instanceof Flyer) {
    echo 'Can fly';
} else {
    echo 'Cannot fly';
}
?>

A Can fly

B No output

C Fatal error

D Cannot fly

💡 Hint

Check if instanceof works with interfaces as well as classes.

The object $bird implements the interface Flyer, so $bird instanceof Flyer returns true and prints 'Can fly'.

❓ Predict Output

advanced

What error or output does this code produce?

Analyze this PHP code and select the correct output or error message.

PHP

<?php
class Vehicle {}
$car = new Vehicle();
if ($car instanceof NonExistentClass) {
    echo 'Yes';
} else {
    echo 'No';
}
?>

A Yes

B Warning: instanceof expects an object

C Fatal error: Class 'NonExistentClass' not found

D No

💡 Hint

What happens if you use instanceof with a class name that does not exist?

Using instanceof with a class name that is not defined causes a fatal error in PHP because the class does not exist.

❓ Predict Output

advanced

What is the output of this code using instanceof with null?

What will this PHP code print?

PHP

<?php
class Person {}
$person = null;
if ($person instanceof Person) {
    echo 'Is a person';
} else {
    echo 'Not a person';
}
?>

A Not a person

B Is a person

C Fatal error

D Warning: instanceof expects an object

💡 Hint

Check how instanceof behaves when the variable is null.

When the variable is null, instanceof returns false without error, so it prints 'Not a person'.

🧠 Conceptual

expert

Which statement about the instanceof operator in PHP is TRUE?

Select the only true statement about the instanceof operator in PHP.

A instanceof can be used to check if a variable is a primitive type like integer or string.

B instanceof returns true if the variable is null and the class name matches any class.

C instanceof can be used to check if a variable is an instance of a class or interface, even if the class or interface does not exist.

D instanceof throws a fatal error if the class or interface name used does not exist in the code.

💡 Hint

Think about what happens if you use instanceof with a class name that PHP does not know.

Using instanceof with a class or interface name that does not exist causes a fatal error in PHP. It cannot check unknown types. It returns false for null variables and cannot check primitive types.

Practice

1. What does the instanceof operator do in PHP?

easy

A. Creates a new instance of a class

B. Checks if an object is an instance of a specific class or its subclass

C. Deletes an object from memory

D. Converts an object to a string

Solution

1. Step 1: Understand the purpose of instanceof

   The instanceof operator is used to check if a variable is an object of a certain class or subclass.

2. Step 2: Differentiate from other operations

   It does not create, delete, or convert objects; it only checks their type.

3. Final Answer:

   Checks if an object is an instance of a specific class or its subclass -> Option B

4. Quick Check:

   instanceof checks object type [OK]

Hint: Remember: instanceof checks object type, not creation or deletion [OK]

Common Mistakes:

• Confusing instanceof with object creation
• Thinking instanceof deletes objects
• Assuming instanceof converts objects to strings

2. Which of the following is the correct syntax to check if $obj is an instance of class Car?

easy

A. if ($obj instance Car) {}

B. if ($obj is Car) {}

C. if (instanceof $obj Car) {}

D. if ($obj instanceof Car) {}

Solution

1. Step 1: Recall correct instanceof syntax

   The correct syntax is: variable instanceof ClassName, without extra keywords or rearrangement.

2. Step 2: Check each option

   if ($obj instanceof Car) {} matches the correct syntax exactly. Others have wrong keywords or order.

3. Final Answer:

   if ($obj instanceof Car) {} -> Option D

4. Quick Check:

   Correct instanceof syntax [OK]

Hint: Syntax is always: variable instanceof ClassName [OK]

Common Mistakes:

• Using 'is' instead of 'instanceof'
• Reversing order of variable and class
• Adding extra keywords inside condition

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

$a = new DateTime();
if ($a instanceof DateTime) {
    echo 'Yes';
} else {
    echo 'No';
}

medium

A. Yes

B. No

C. Error

D. Nothing

Solution

1. Step 1: Understand object creation

   $a is created as a new instance of the DateTime class.

2. Step 2: Check instanceof condition

   The condition $a instanceof DateTime is true because $a is exactly a DateTime object.

3. Final Answer:

   Yes -> Option A

4. Quick Check:

   Object is DateTime = Yes [OK]

Hint: If object matches class, instanceof returns true [OK]

Common Mistakes:

• Assuming instanceof returns false for exact class
• Expecting syntax error from instanceof
• Ignoring else branch output

4. Identify the error in this PHP code snippet:

class Animal {}
class Dog extends Animal {}
$dog = new Dog();
if ($dog instanceof 'Animal') {
    echo 'It is an animal';
}

medium

A. Missing semicolon after class definition

B. Incorrect object creation syntax

C. Using quotes around class name in instanceof

D. Using instanceof with a variable instead of object

Solution

1. Step 1: Check instanceof usage

   The instanceof operator expects a class name without quotes, not a string.

2. Step 2: Verify other parts

   Classes and object creation are correct; semicolons are present.

3. Final Answer:

   Using quotes around class name in instanceof -> Option C

4. Quick Check:

   Class name must be unquoted in instanceof [OK]

Hint: Do not put quotes around class names in instanceof [OK]

Common Mistakes:

• Putting class names in quotes inside instanceof
• Confusing instanceof with string comparison
• Missing semicolons elsewhere (not in this code)

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

class Vehicle {}
class Car extends Vehicle {}
class Bike extends Vehicle {}

function checkVehicle($obj) {
    if ($obj instanceof Car) {
        return 'Car';
    } elseif ($obj instanceof Vehicle) {
        return 'Vehicle';
    } else {
        return 'Unknown';
    }
}

$myBike = new Bike();
echo checkVehicle($myBike);

hard

A. Vehicle

B. Unknown

C. Car

D. Error

Solution

1. Step 1: Analyze object type

   $myBike is an instance of Bike, which extends Vehicle.

2. Step 2: Follow function logic

   First condition checks if $obj is Car - false. Second checks if $obj is Vehicle - true because Bike is subclass of Vehicle.

3. Final Answer:

   Vehicle -> Option A

4. Quick Check:

   Bike is a Vehicle, not Car = Vehicle [OK]

Hint: Check subclass before superclass in instanceof chains [OK]

Common Mistakes:

• Expecting 'Car' for Bike object
• Ignoring subclass relationship in instanceof
• Assuming instanceof only matches exact class