Intersection types in PHP - Step-by-Step Execution

Concept Flow - Intersection types

Define Interfaces

↓

Create Class implementing both

↓

Use Intersection Type in Function

↓

Pass Object to Function

↓

Function accepts only if object matches ALL types

Intersection types require a value to satisfy multiple type constraints at once, like an object implementing all specified interfaces.

Execution Sample

PHP

<?php
interface A { function a(); }
interface B { function b(); }
class C implements A, B {
  function a() { echo "A"; }
  function b() { echo "B"; }
}
function test(A&B $obj) {
  $obj->a(); $obj->b();
}
$obj = new C();
test($obj);
?>

This code defines two interfaces and a class implementing both, then a function requiring an object that implements both interfaces (intersection type).

Execution Table

Step	Action	Evaluation	Result
1	Define interface A	interface A with method a()	Interface A created
2	Define interface B	interface B with method b()	Interface B created
3	Define class C implements A and B	Class C has methods a() and b()	Class C created
4	Create function test with parameter type A&B	Parameter must implement both A and B	Function test created
5	Create object $obj = new C()	Object $obj is instance of C	Object $obj created
6	Call test($obj)	Check if $obj implements A and B	Passes, function runs
7	Inside test: call $obj->a()	Calls method a()	Outputs 'A'
8	Inside test: call $obj->b()	Calls method b()	Outputs 'B'
9	Function test ends	All calls successful	Output: 'AB'

💡 Function test accepts only objects implementing both interfaces A and B; $obj meets this, so execution completes.

Variable Tracker

Variable	Start	After Step 5	After Step 6	Final
$obj	undefined	Instance of C	Passed to test()	Instance of C

Key Moments - 2 Insights

Why does the function parameter use A&B instead of just A or B?

Because the function requires an object that implements both interfaces A and B simultaneously, not just one. See execution_table step 4 and 6 where the intersection type enforces this.

What happens if we pass an object that implements only A or only B?

The function call will fail because the intersection type requires both. This is shown in execution_table step 6 where the check ensures the object implements both interfaces.

Visual Quiz - 3 Questions

Look at the execution_table at step 6, what is checked before the function runs?

A If $obj implements both interfaces A and B

B If $obj implements either interface A or B

C If $obj is a subclass of C

D If $obj is null

💡 Hint

Check the 'Evaluation' column at step 6 in execution_table.

At which step does the program output the letter 'B'?

A Step 7

B Step 8

C Step 6

D Step 9

💡 Hint

Look at the 'Action' and 'Result' columns for output in execution_table steps 7 and 8.

If class C did not implement interface B, what would happen at step 6?

A Function test would throw an error at step 7

B Function test would accept $obj anyway

C Function test would reject $obj due to missing interface B

D Nothing would change

💡 Hint

Refer to the intersection type requirement in execution_table step 4 and 6.

Concept Snapshot

Intersection types in PHP require a value to satisfy multiple types at once.
Syntax: function f(A&B $obj) means $obj must implement both A and B.
Useful for enforcing multiple interface implementations.
If the object misses any type, PHP throws a TypeError.
Allows safer, clearer code contracts.

Full Transcript

This example shows how PHP intersection types work by defining two interfaces A and B, and a class C that implements both. A function test requires an argument that implements both interfaces, using the syntax A&B. When an object of class C is passed, the function calls methods from both interfaces successfully, outputting 'AB'. The execution table traces each step from defining interfaces and class, creating the object, checking the intersection type, and calling methods. Key moments clarify why both interfaces are required and what happens if an object does not meet the intersection. The visual quiz tests understanding of type checks and output steps. Intersection types help ensure objects meet multiple type requirements simultaneously, improving code safety and clarity.