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.

Practice

1. What is the main purpose of an interface in PHP?

easy

A. To execute code automatically when a program runs

B. To store variables and their values

C. To define method names without their code, ensuring classes implement them

D. To create objects directly

Solution

1. Step 1: Understand what an interface is

   An interface in PHP only declares method names without any code inside them.

2. Step 2: Understand the purpose of interfaces

   Interfaces ensure that any class implementing them must write the declared methods, helping organize code.

3. Final Answer:

   To define method names without their code, ensuring classes implement them -> Option C

4. Quick Check:

   Interface purpose = method declaration only [OK]

Hint: Interfaces declare methods only, no code inside [OK]

Common Mistakes:

• Thinking interfaces store variables
• Believing interfaces create objects
• Confusing interfaces with classes

2. Which of the following is the correct way to declare an interface named Vehicle in PHP?

easy

A. class Vehicle { public function start(); }

B. interface Vehicle { public function start(); }

C. interface Vehicle { function start() {} }

D. interface Vehicle { public function start() {} }

Solution

1. Step 1: Recall interface method declaration rules

   In interfaces, methods are declared without code blocks (no curly braces with code).

2. Step 2: Check each option

   'interface Vehicle { public function start(); }' declares interface and method without code, which is correct. Declarations with method bodies '{}' are wrong. Using 'class' instead of 'interface' is incorrect.

3. Final Answer:

   interface Vehicle { public function start(); } -> Option B

4. Quick Check:

   Interface methods have no body [OK]

Hint: Interface methods end with semicolon, no braces [OK]

Common Mistakes:

• Adding method bodies inside interfaces
• Using class keyword instead of interface
• Omitting semicolon after method declaration

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

interface Animal {
  public function sound();
}

class Dog implements Animal {
  public function sound() {
    return "Bark";
  }
}

$dog = new Dog();
echo $dog->sound();

medium

A. Bark

B. sound

C. Animal

D. Fatal error

Solution

1. Step 1: Understand interface and class implementation

   The interface Animal declares method sound(). Dog class implements Animal and defines sound() returning "Bark".

2. Step 2: Check the output of calling $dog->sound()

   Calling $dog->sound() returns "Bark" which is then echoed.

3. Final Answer:

   Bark -> Option A

4. Quick Check:

   Implemented method returns "Bark" [OK]

Hint: Implemented methods run normally, output their return [OK]

Common Mistakes:

• Expecting interface name as output
• Confusing method name with output
• Thinking code causes error without implementation

4. Identify the error in this PHP code:

interface Shape {
  public function area();
}

class Circle implements Shape {
  public function area() {
    return 3.14 * 5 * 5;
  }

  public function perimeter() {
    return 2 * 3.14 * 5;
  }
}

$circle = new Circle();
echo $circle->area();

medium

A. No error, code runs and outputs area

B. Missing implementation of perimeter() in interface

C. Interface method area() has no body, causing error

D. Class Circle must implement all interface methods

Solution

1. Step 1: Check interface and class methods

   Interface Shape declares area(). Circle implements area() correctly and adds extra method perimeter() which is allowed.

2. Step 2: Verify if code runs without error

   Since all interface methods are implemented, extra methods are allowed. Code outputs area value.

3. Final Answer:

   No error, code runs and outputs area -> Option A

4. Quick Check:

   Extra methods allowed, interface methods implemented [OK]

Hint: Extra methods in class are allowed beyond interface [OK]

Common Mistakes:

• Thinking extra methods must be in interface
• Confusing method body rules in interface
• Assuming missing interface methods cause error here

5. You have two interfaces:

interface Printer {
  public function printDocument();
}

interface Scanner {
  public function scanDocument();
}

How can you create a class AllInOne that implements both interfaces and provides method bodies?

hard

A. class AllInOne implements Scanner {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

B. class AllInOne implements Printer {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

C. class AllInOne {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

D. class AllInOne implements Printer, Scanner {
public function printDocument() { return "Printing..."; }
public function scanDocument() { return "Scanning..."; }
}

Solution

1. Step 1: Understand multiple interface implementation

   PHP allows a class to implement multiple interfaces by listing them separated by commas.

2. Step 2: Check which option correctly implements both interfaces

   class AllInOne implements Printer, Scanner { public function printDocument() { return "Printing..."; } public function scanDocument() { return "Scanning..."; } } implements both Printer and Scanner and defines both required methods. Others miss one interface or do not implement any.

3. Final Answer:

   class AllInOne implements Printer, Scanner { public function printDocument() { return "Printing..."; } public function scanDocument() { return "Scanning..."; } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with commas [OK]

Hint: Use commas to implement multiple interfaces in one class [OK]

Common Mistakes:

• Implementing only one interface when multiple required
• Forgetting to implement all interface methods
• Not using commas to separate interfaces