Intersection types in practice in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of intersection type function call

What is the output of this PHP code using intersection types?

PHP

<?php
interface A { public function foo(): string; }
interface B { public function bar(): string; }
class C implements A, B {
    public function foo(): string { return 'foo'; }
    public function bar(): string { return 'bar'; }
}
function test( A&B $obj ): string {
    return $obj->foo() . '-' . $obj->bar();
}
$c = new C();
echo test($c);
?>

A SyntaxError

B foobar

C foo-barbar

D foo-bar

💡 Hint

Remember that the function requires an object implementing both interfaces A and B.

The function test accepts an object that implements both interfaces A and B. The class C implements both, so calling test($c) returns foo-bar.

❓ Predict Output

intermediate

Return type with intersection types

What will this PHP code output?

PHP

<?php
interface X { public function x(): string; }
interface Y { public function y(): string; }
class Z implements X, Y {
    public function x(): string { return 'X'; }
    public function y(): string { return 'Y'; }
}
function create(): X&Y {
    return new Z();
}
$obj = create();
echo $obj->x() . $obj->y();
?>

A XY

B X Y

C SyntaxError

D Fatal error: Return type mismatch

💡 Hint

The function returns an object implementing both interfaces X and Y.

The function create returns an instance of Z which implements both X and Y. So calling $obj->x() and $obj->y() outputs XY.

🔧 Debug

advanced

Identify the error with intersection types

What error will this PHP code produce?

PHP

<?php
interface M { public function m(): string; }
interface N { public function n(): string; }
class P implements M {
    public function m(): string { return 'm'; }
}
function callMN(M&N $obj): string {
    return $obj->m() . $obj->n();
}
$p = new P();
echo callMN($p);
?>

A Fatal error: Uncaught TypeError: Argument 1 passed to callMN() must implement interface N

B SyntaxError

C mnull

D Call to undefined method P::n()

💡 Hint

Check if the object passed implements both interfaces M and N.

The function callMN requires an object implementing both M and N. The class P only implements M. Passing $p causes a TypeError because P does not implement N.

📝 Syntax

advanced

Syntax error with intersection types

Which option shows the correct syntax to declare a function parameter with intersection types in PHP?

A function example(A && B $param): void {}

B function example(A & B $param): void {}

C function example(A&B $param): void {}

D function example(A | B $param): void {}

💡 Hint

Intersection types use a single ampersand without spaces.

The correct syntax for intersection types in PHP is A&B without spaces. Option C is correct. Option C has spaces which cause syntax error. Option C uses double ampersand which is invalid. Option C uses pipe which is union type, not intersection.

🚀 Application

expert

Using intersection types with traits and interfaces

Given the code below, what will be the output?

PHP

<?php
interface I1 { public function get(): string; }
interface I2 { public function get(): string; }
trait T1 { public function get(): string { return 'Trait'; } }
class MyClass implements I1, I2 {
    use T1;
}
function show(I1&I2 $obj): string {
    return $obj->get();
}
$instance = new MyClass();
echo show($instance);
?>

A Interface method get() must be public

B Trait

C SyntaxError

D Fatal error: Class MyClass must implement method get()

💡 Hint

Traits can provide method implementations for interfaces.

The class MyClass implements both interfaces I1 and I2 and uses trait T1 which provides the get() method. So calling show($instance) outputs Trait.

Practice

1. What does an intersection type like A&B mean in PHP?

easy

A. The value must be type B only.

B. The value can be either type A or type B.

C. The value must satisfy both type A and type B simultaneously.

D. The value must be type A only.

Solution

1. Step 1: Understand intersection type concept

   Intersection types require a value to meet all listed types at once, not just one.

2. Step 2: Apply to A&B

   The value must be both type A and type B simultaneously.

3. Final Answer:

   The value must satisfy both type A and type B simultaneously. -> Option C

4. Quick Check:

   Intersection means AND, so both types required [OK]

Hint: Intersection means all types must be met together [OK]

Common Mistakes:

• Confusing intersection (&) with union (|)
• Thinking it means either type A or B
• Assuming it means only one type

2. Which of the following is the correct syntax for declaring a function parameter with an intersection type of interfaces Countable and Iterator?

easy

A. function process(Countable&Iterator $obj) {}

B. function process(Countable|Iterator $obj) {}

C. function process(Countable, Iterator $obj) {}

D. function process(Countable & Iterator $obj) {}

Solution

1. Step 1: Recall intersection type syntax in PHP

   PHP uses the ampersand (&) without spaces and escaped as & in HTML to declare intersection types.

2. Step 2: Check each option

   function process(Countable&Iterator $obj) {} uses Countable&Iterator which is correct syntax for intersection types in PHP.

3. Final Answer:

   function process(Countable&Iterator $obj) {} -> Option A

4. Quick Check:

   Intersection uses & without spaces [OK]

Hint: Use & with no spaces for intersection types [OK]

Common Mistakes:

• Using | instead of & for intersection
• Adding spaces around &
• Using comma instead of &

3. Given the following code, what will be the output?

interface A { public function foo(): string; }
interface B { public function bar(): string; }

class C implements A, B {
  public function foo(): string { return 'foo'; }
  public function bar(): string { return 'bar'; }
}

function test(A&B $obj) {
  echo $obj->foo() . ' ' . $obj->bar();
}

$c = new C();
test($c);

medium

A. foo bar

B. foo

C. bar

D. TypeError at runtime

Solution

1. Step 1: Understand class C implements both interfaces

   Class C implements both A and B, so it has methods foo() and bar().

2. Step 2: Call test with intersection type parameter

   Function test requires an object that is both A and B, which $c is. It calls foo() and bar() and echoes their results separated by space.

3. Final Answer:

   foo bar -> Option A

4. Quick Check:

   Object meets both interfaces, so output is 'foo bar' [OK]

Hint: Check if object implements all interfaces in intersection [OK]

Common Mistakes:

• Expecting only one method output
• Thinking intersection causes error if both interfaces implemented
• Confusing union with intersection types

4. What is the error in this code snippet?

interface X { public function x(): void; }
interface Y { public function y(): void; }

function example(X&Y $obj) {
  $obj->x();
  $obj->y();
}

class Z implements X {
  public function x(): void {}
}

$z = new Z();
example($z);

medium

A. No error, code runs fine.

B. TypeError because $z does not implement interface Y.

C. Syntax error in function declaration.

D. Fatal error: method y() not found in class Z.

Solution

1. Step 1: Check class Z interfaces

   Class Z implements only interface X, not Y.

2. Step 2: Analyze function parameter type

   Function example requires an object implementing both X and Y (intersection). Passing $z violates this.

3. Final Answer:

   TypeError because $z does not implement interface Y. -> Option B

4. Quick Check:

   Intersection requires all interfaces, missing one causes TypeError [OK]

Hint: All interfaces in intersection must be implemented [OK]

Common Mistakes:

• Assuming partial interface implementation is enough
• Confusing syntax error with type error
• Thinking missing method causes fatal error instead of TypeError

5. You want to write a function that accepts an object which must be both JsonSerializable and Countable. You also want to ensure the object has a method customMethod(). Which of the following is the best way to type hint this in PHP 8.1+?

hard

A. function process(JsonSerializable|Countable $obj) { $obj->customMethod(); }

B. function process(JsonSerializable&Countable&CustomInterface $obj) {}

C. function process(object $obj) { $obj->customMethod(); }

D. function process(JsonSerializable&Countable $obj) { if (method_exists($obj, 'customMethod')) { $obj->customMethod(); } }

Solution

1. Step 1: Understand intersection type usage

   Use & to require both JsonSerializable and Countable interfaces.

2. Step 2: Handle custom method not in interfaces

   Since customMethod() is not part of these interfaces, check its existence with method_exists before calling.

3. Final Answer:

   function process(JsonSerializable&Countable $obj) { if (method_exists($obj, 'customMethod')) { $obj->customMethod(); } } -> Option D

4. Quick Check:

   Intersection + runtime check for extra method [OK]

Hint: Use & for interfaces and method_exists for extra methods [OK]

Common Mistakes:

• Using union (|) instead of intersection (&)
• Assuming method in interfaces without checking
• Ignoring type safety by using object without interfaces