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.