Multiple interface implementation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multiple interface implementation

What is the output of this PHP code that implements two interfaces in one class?

PHP

<?php
interface A {
    public function foo();
}
interface B {
    public function bar();
}
class C implements A, B {
    public function foo() {
        echo "foo called\n";
    }
    public function bar() {
        echo "bar called\n";
    }
}
$obj = new C();
$obj->foo();
$obj->bar();
?>

A

foo called
bar called

B

foo called

C

bar called

D Fatal error: Class C must implement method bar()

💡 Hint

Remember that a class implementing multiple interfaces must define all their methods.

Class C implements both interfaces A and B, so it must define both foo() and bar(). The code calls both methods, so both outputs appear.

❓ Predict Output

intermediate

Which method is called from which interface?

Given this PHP code with two interfaces and a class implementing both, what is the output?

PHP

<?php
interface X {
    public function test();
}
interface Y {
    public function test();
}
class Z implements X, Y {
    public function test() {
        echo "test method from Z\n";
    }
}
$z = new Z();
$z->test();
?>

A

test method from Z

B Fatal error: Class Z must implement method test() from interface Y

C

test method from X

D No output

💡 Hint

One method can satisfy multiple interfaces if signatures match.

Both interfaces declare the same method name and signature. The class implements one method that satisfies both interfaces. Calling test() outputs the defined string.

❓ Predict Output

advanced

Output when interface methods have different signatures

What happens when a class tries to implement two interfaces with methods of the same name but different parameters?

PHP

<?php
interface I1 {
    public function run(int $a);
}
interface I2 {
    public function run(string $b);
}
class Test implements I1, I2 {
    public function run($param) {
        echo gettype($param) . "\n";
    }
}
$test = new Test();
$test->run(5);
$test->run("hello");
?>

A

string
string

B

integer
integer

C

integer
string

D Fatal error: Declaration of Test::run() must be compatible with both interfaces

💡 Hint

PHP requires method signatures to be compatible when implementing multiple interfaces.

The class method run() cannot satisfy both interface signatures because one expects int and the other string. This causes a fatal error.

❓ Predict Output

advanced

Output of interface constants in multiple interfaces

What is the output of this PHP code using constants from multiple interfaces implemented by one class?

PHP

<?php
interface First {
    const VALUE = 10;
}
interface Second {
    const VALUE = 20;
}
class Demo implements First, Second {
    public function show() {
        echo self::VALUE . "\n";
    }
}
$obj = new Demo();
$obj->show();
?>

A

0

B

20

C Fatal error: Demo::VALUE is ambiguous

D

10

💡 Hint

When interfaces have constants with the same name, the class must resolve ambiguity.

The class Demo inherits VALUE constant from both interfaces. Using self::VALUE is ambiguous and causes a fatal error.

🧠 Conceptual

expert

Why must a class implement all interface methods?

In PHP, why does a class that implements multiple interfaces have to define all methods declared in those interfaces?

A Because PHP automatically provides default implementations for interface methods.

B Because interfaces define a contract that the class must fulfill to be considered valid.

C Because interfaces are optional and do not require method definitions.

D Because classes can only implement one interface at a time.

💡 Hint

Think about what interfaces represent in object-oriented programming.

Interfaces specify methods that a class promises to implement. This contract ensures consistent behavior and allows polymorphism.