Interface declaration and implementation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of implemented interface method

What is the output of this PHP code when run?

PHP

<?php
interface Talker {
    public function sayHello();
}

class Person implements Talker {
    public function sayHello() {
        echo "Hello from Person!";
    }
}

$p = new Person();
$p->sayHello();
?>

A Hello from Person!

B Fatal error: Class Person must implement method sayHello()

C Hello from Talker!

D No output

💡 Hint

Check how the interface method is implemented in the class.

The class Person implements the interface Talker and defines the sayHello() method which prints 'Hello from Person!'. So the output is exactly that string.

❓ Predict Output

intermediate

Output when interface method is missing

What error or output will this PHP code produce?

PHP

<?php
interface Flyer {
    public function fly();
}

class Bird implements Flyer {
    // Missing fly() method
}

$b = new Bird();
$b->fly();
?>

A No output

B Call to undefined method Bird::fly()

C Fatal error: Class Bird contains 1 abstract method and must therefore be declared abstract or implement the remaining methods

D Fly method called

💡 Hint

Check if the class implements all interface methods.

Since Bird does not implement the required fly() method from Flyer interface, PHP throws a fatal error saying Bird must implement the remaining methods or be declared abstract.

🔧 Debug

advanced

Identify the error in interface implementation

This PHP code tries to implement an interface but causes an error. What is the cause?

PHP

<?php
interface Runner {
    public function run(int $speed);
}

class Athlete implements Runner {
    public function run($speed) {
        echo "Running at speed $speed";
    }
}

$a = new Athlete();
$a->run(10);
?>

A Warning: Argument 1 passed to run() must be of the type int

B Fatal error due to missing type hint in method parameter

C No error, code runs and outputs 'Running at speed 10'

D Parse error: syntax error in interface declaration

💡 Hint

Check if parameter types must match exactly in interface and implementation.

In PHP, when implementing an interface, the method signature must be compatible, including parameter type hints. Athlete::run($speed) omits the 'int' type required by Runner::run(int $speed), causing a fatal error: 'Declaration of Athlete::run($speed) must be compatible with Runner::run(int $speed)'.

📝 Syntax

advanced

Which code causes a syntax error?

Which of these PHP code snippets will cause a syntax error?

A ?php interface A { public function foo(); } class B implements A { public function foo() { echo 'Hi'; } } ?>

B <?php interface A { public function foo(); } class B implements A { public function foo() { echo 'Hi'; } } ?>

C >? } } ;'iH' ohce { )(oof noitcnuf cilbup { A stnemelpmi B ssalc } ;)(oof noitcnuf cilbup { A ecafretni php?<

D <?php interface A { public function foo(); } class B implements A { public function foo() { echo 'Hi' } } ?>

💡 Hint

Look carefully at the semicolons inside the method.

Option D is missing a semicolon after echo 'Hi', causing a syntax error. The others have correct syntax.

🚀 Application

expert

Number of methods in implemented interface

Given this PHP code, how many methods must class C implement to satisfy all interfaces?

PHP

<?php
interface X {
    public function a();
}
interface Y {
    public function b();
    public function c();
}
interface Z extends X, Y {
    public function d();
}
class C implements Z {
    public function a() {}
    public function b() {}
    public function c() {}
    public function d() {}
}
?>

A 4

B 1

C 2

D 3

💡 Hint

Remember that interface Z extends X and Y, so it includes all their methods.

Interface Z extends X and Y, so it requires methods a(), b(), c(), and d(). Class C must implement all four methods.