Intersection types in PHP - Mini Project: Build & Apply

Intersection Types in PHP

📖 Scenario: Imagine you are building a simple system to manage different types of workers in a company. Some workers can both write reports and attend meetings. You want to make sure that certain functions only accept workers who can do both tasks.

🎯 Goal: You will create interfaces for ReportWriter and MeetingAttendee, then create a class that implements both. Finally, you will write a function that accepts only objects that satisfy both interfaces using intersection types.

📋 What You'll Learn

Create two interfaces: ReportWriter and MeetingAttendee with one method each.

Create a class Employee that implements both interfaces.

Write a function handleWorker that accepts a parameter with an intersection type of ReportWriter&MeetingAttendee.

Call the function with an Employee object and print messages from both methods.

💡 Why This Matters

🌍 Real World

Intersection types help ensure that objects passed to functions meet multiple requirements, useful in complex systems where roles overlap.

💼 Career

Understanding intersection types is important for writing clear, type-safe code in PHP, especially in large projects or when using modern PHP features.

1

Create interfaces for report writing and meeting attendance

Create two interfaces called ReportWriter and MeetingAttendee. In ReportWriter, declare a public method writeReport(). In MeetingAttendee, declare a public method attendMeeting().

PHP

<?php

// Create interface ReportWriter with method writeReport()
// Create interface MeetingAttendee with method attendMeeting()

Hint

Use the interface keyword to create interfaces. Each interface should have one method declaration without a body.

2

Create a class that implements both interfaces

Create a class called Employee that implements both ReportWriter and MeetingAttendee. Implement the methods writeReport() and attendMeeting() to return the strings "Report written." and "Meeting attended." respectively.

PHP

<?php

interface ReportWriter {
    public function writeReport(): string;
}

interface MeetingAttendee {
    public function attendMeeting(): string;
}

// Create class Employee implementing ReportWriter and MeetingAttendee
// Implement writeReport() and attendMeeting() methods

Hint

Use implements keyword to implement multiple interfaces. Each method must return the exact string.

3

Write a function using intersection types

Write a function called handleWorker that accepts one parameter with the intersection type ReportWriter&MeetingAttendee. Inside the function, call writeReport() and attendMeeting() on the parameter and store their results in variables $report and $meeting.

PHP

<?php

interface ReportWriter {
    public function writeReport(): string;
}

interface MeetingAttendee {
    public function attendMeeting(): string;
}

class Employee implements ReportWriter, MeetingAttendee {
    public function writeReport(): string {
        return "Report written.";
    }

    public function attendMeeting(): string {
        return "Meeting attended.";
    }
}

// Write function handleWorker with parameter type ReportWriter&MeetingAttendee
// Call writeReport() and attendMeeting() on the parameter

Hint

Use the intersection type ReportWriter&MeetingAttendee in the function parameter to require both interfaces.

4

Call the function and print the results

Inside the handleWorker function, add two echo statements to print $report and $meeting each followed by a newline. Then, create an Employee object called $employee and call handleWorker($employee).

PHP

writeReport();
    $meeting = $worker->attendMeeting();
    // Print $report and $meeting with echo
}

// Create Employee object and call handleWorker

Hint

Use echo to print the strings with a newline character \n. Create the object and call the function after it.

Practice

1. What does an intersection type in PHP represent?
function example(SomeType&OtherType $param) {}

easy

A. A type that requires the value to satisfy both SomeType and OtherType.

B. A type that accepts either SomeType or OtherType.

C. A type that only accepts SomeType but ignores OtherType.

D. A type that combines SomeType and OtherType into a new class.

Solution

1. Step 1: Understand intersection type syntax

   In PHP, the & symbol between types means the value must satisfy all listed types simultaneously.

2. Step 2: Interpret the function parameter

   The parameter type SomeType&OtherType means the argument must be both SomeType and OtherType at the same time.

3. Final Answer:

   A type that requires the value to satisfy both SomeType and OtherType. -> Option A

4. Quick Check:

   Intersection type = must satisfy all types [OK]

Hint: Remember & means AND, so all types must match [OK]

Common Mistakes:

• Confusing & with | (union type)
• Thinking it creates a new combined class
• Assuming it accepts either type instead of both

2. Which of the following is the correct syntax for declaring an intersection type in a function parameter in PHP?

easy

A. function test(A & B $param) {}

B. function test(A | B $param) {}

C. function test(A, B $param) {}

D. function test(A && B $param) {}

Solution

1. Step 1: Recall intersection type syntax

   PHP uses a single ampersand & to declare intersection types between multiple types.

2. Step 2: Check each option

   function test(A & B $param) {} uses & correctly. function test(A | B $param) {} uses | which is union type. function test(A, B $param) {} uses comma which is invalid. function test(A && B $param) {} uses && which is not valid syntax.

3. Final Answer:

   function test(A & B $param) {} -> Option A

4. Quick Check:

   Intersection type uses & symbol [OK]

Hint: Use single & between types for intersection [OK]

Common Mistakes:

• Using | instead of &
• Using commas to separate types
• Using && instead of &

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

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. TypeError at runtime

B. foo

C. bar

D. foo bar

Solution

1. Step 1: Understand class and interfaces

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

2. Step 2: Analyze function test parameter and call

   The function test requires an argument that satisfies both A and B (intersection type). Object $c satisfies both. The function echoes foo() and bar() results concatenated with a space.

3. Final Answer:

   foo bar -> Option D

4. Quick Check:

   Object satisfies both interfaces, prints both methods [OK]

Hint: Intersection means object must have all methods used [OK]

Common Mistakes:

• Expecting only one method output
• Thinking it causes a TypeError
• Confusing intersection with union types

4. Identify the error in this PHP code using intersection types:

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

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

function foo(X & Y $param) {
  $param->x();
  $param->y();
}

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

medium

A. Function foo has wrong parameter syntax for intersection types.

B. Method y() is missing in function foo.

C. Class Z does not implement interface Y, so it cannot satisfy X & Y.

D. No error, code runs fine.

Solution

1. Step 1: Check class Z implementation

   Class Z implements only interface X, not Y, so it does not satisfy the intersection type X & Y.

2. Step 2: Analyze function call with $z

   Function foo requires a parameter that implements both X and Y. Passing $z causes a TypeError because $z lacks Y.

3. Final Answer:

   Class Z does not implement interface Y, so it cannot satisfy X & Y. -> Option C

4. Quick Check:

   Intersection requires all interfaces implemented [OK]

Hint: Check all interfaces are implemented for intersection [OK]

Common Mistakes:

• Assuming partial interface implementation is enough
• Thinking syntax is wrong instead of logic
• Ignoring missing methods from interfaces

5. Given these interfaces and classes:

interface Logger { public function log(string $msg): void; }
interface FileWriter { public function write(string $data): void; }

class FileLogger implements Logger, FileWriter {
  public function log(string $msg): void { echo "Log: $msg\n"; }
  public function write(string $data): void { echo "Write: $data\n"; }
}

function process(Logger & FileWriter $obj, string $message) {
  $obj->log($message);
  $obj->write($message);
}

$fl = new FileLogger();
process($fl, "Hello World");

What will be the output and why is intersection type useful here?

hard

A. Runtime error because intersection types cannot be used with classes.

B. Output:Log: Hello WorldWrite: Hello WorldIntersection ensures $obj has both log() and write() methods.

C. Output:Log: Hello WorldOnly log() is called because intersection ignores write().

D. Output:Write: Hello WorldOnly write() is called because log() is ignored.

Solution

1. Step 1: Understand class and interfaces

   FileLogger implements both Logger and FileWriter, so it has both log() and write() methods.

2. Step 2: Analyze function process behavior

   The function requires an object that satisfies both Logger and FileWriter (intersection type). It calls both log() and write() methods on the object.

3. Step 3: Predict output

   Calling process with $fl outputs:
   Log: Hello World
   Write: Hello World

4. Final Answer:

   Output:
   Log: Hello World
   Write: Hello World
   Intersection ensures $obj has both log() and write() methods. -> Option B

5. Quick Check:

   Intersection type ensures all required methods exist [OK]

Hint: Intersection guarantees all methods exist on object [OK]

Common Mistakes:

• Thinking only one interface method is called
• Believing intersection types cause runtime errors
• Ignoring that both methods are called