Multiple interface implementation in PHP - Step-by-Step Execution

Concept Flow - Multiple interface implementation

Define Interface A

↓

Define Interface B

↓

Create Class implementing A and B

↓

Implement all methods from A and B

↓

Create object of Class

↓

Call methods from both interfaces

The class promises to follow rules from both interfaces by implementing all their methods.

Execution Sample

PHP

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

This code shows a class implementing two interfaces and calling their methods.

Execution Table

Step	Action	Evaluation	Result
1	Define interface A with method foo()	Interface A created	Interface A ready
2	Define interface B with method bar()	Interface B created	Interface B ready
3	Create class MyClass implementing A and B	Class MyClass created	Class ready with interfaces A and B
4	Implement method foo() in MyClass	Method foo() defined	foo() returns 'foo called'
5	Implement method bar() in MyClass	Method bar() defined	bar() returns 'bar called'
6	Create object obj of MyClass	Object created	obj is instance of MyClass
7	Call obj->foo()	Calls foo()	Output: 'foo called'
8	Call obj->bar()	Calls bar()	Output: 'bar called'
9	End of script	No more code	Execution stops

💡 All interface methods implemented; object methods called; script ends

Variable Tracker

Variable	Start	After Step 6	After Step 7	After Step 8	Final
obj	undefined	instance of MyClass	instance of MyClass	instance of MyClass	instance of MyClass

Key Moments - 2 Insights

Why must MyClass implement all methods from both interfaces A and B?

Because PHP requires that a class implementing interfaces must define all their methods, as shown in steps 4 and 5 of the execution_table.

What happens if MyClass misses implementing one method from an interface?

PHP will give an error and stop execution, because the class does not fulfill the interface contract, which is why steps 4 and 5 are important.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after calling obj->foo() at step 7?

A No output

B "bar called"

C "foo called"

D Error

💡 Hint

Check the 'Result' column at step 7 in the execution_table

At which step does the object obj get created?

A Step 6

B Step 4

C Step 7

D Step 3

💡 Hint

Look for 'Create object obj' in the 'Action' column of execution_table

If MyClass did not implement method bar(), what would happen?

A The script runs normally

B PHP throws an error at class creation

C Only foo() works, bar() is ignored

D The object cannot be created

💡 Hint

Refer to key_moments about missing method implementation consequences

Concept Snapshot

Multiple interface implementation in PHP:
- Use 'implements' keyword with comma-separated interfaces.
- Class must define all methods from all interfaces.
- Allows one class to follow multiple contracts.
- Example: class MyClass implements A, B { ... }
- Missing methods cause errors.

Full Transcript

This example shows how a PHP class can implement multiple interfaces by listing them after the 'implements' keyword. Each interface defines methods that the class must provide. The class MyClass implements interfaces A and B, so it defines both foo() and bar() methods. An object of MyClass is created and both methods are called, producing outputs 'foo called' and 'bar called'. If any method from the interfaces is missing, PHP will throw an error. This ensures the class follows all interface rules.

Practice

1. What does it mean when a PHP class implements multiple interfaces separated by commas?

easy

A. The class can only use one interface at a time.

B. The class inherits properties from multiple parent classes.

C. The class automatically creates methods from the interfaces.

D. The class promises to have all methods defined in all those interfaces.

Solution

1. Step 1: Understand interface implementation

   When a class implements interfaces, it promises to include all methods declared in those interfaces.

2. Step 2: Multiple interfaces separated by commas

   Listing interfaces separated by commas means the class must implement all methods from each interface.

3. Final Answer:

   The class promises to have all methods defined in all those interfaces. -> Option D

4. Quick Check:

   Multiple interfaces = all methods implemented [OK]

Hint: Remember: commas mean all interfaces must be implemented [OK]

Common Mistakes:

• Thinking interfaces provide properties
• Confusing interfaces with class inheritance
• Believing methods are auto-created

2. Which of the following is the correct syntax for a PHP class implementing two interfaces named FirstInterface and SecondInterface?

easy

A. class MyClass extends FirstInterface, SecondInterface {}

B. class MyClass implements FirstInterface, SecondInterface {}

C. class MyClass implements FirstInterface SecondInterface {}

D. class MyClass uses FirstInterface, SecondInterface {}

Solution

1. Step 1: Identify correct keyword for interfaces

   PHP uses the keyword implements to indicate a class uses interfaces.

2. Step 2: Check syntax for multiple interfaces

   Multiple interfaces are listed separated by commas after implements.

3. Final Answer:

   class MyClass implements FirstInterface, SecondInterface {} -> Option B

4. Quick Check:

   implements + comma-separated interfaces = correct syntax [OK]

Hint: Use 'implements' and commas to list interfaces [OK]

Common Mistakes:

• Using 'extends' instead of 'implements' for interfaces
• Missing commas between interface names
• Using 'uses' keyword incorrectly

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

interface A { public function foo(); }
interface B { public function bar(); }
class Test implements A, B {
  public function foo() { return 'foo'; }
  public function bar() { return 'bar'; }
}
$obj = new Test();
echo $obj->foo() . ' ' . $obj->bar();

medium

A. foo bar

B. foobar

C. Syntax error

D. Runtime error

Solution

1. Step 1: Check interface methods implemented

   The class Test implements both interfaces A and B and defines both methods foo() and bar() correctly.

2. Step 2: Understand output of method calls

   Calling foo() returns 'foo' and bar() returns 'bar'. Echo concatenates them with a space.

3. Final Answer:

   foo bar -> Option A

4. Quick Check:

   Implemented methods return strings combined with space [OK]

Hint: Check all interface methods are implemented to avoid errors [OK]

Common Mistakes:

• Forgetting to implement all interface methods
• Missing space in echo output
• Expecting concatenation without space

4. Identify the error in this PHP code:

interface X { public function methodX(); }
interface Y { public function methodY(); }
class Demo implements X, Y {
  public function methodX() { return 'X'; }
}

medium

A. Interface Y cannot be implemented with interface X.

B. Class Demo should extend interfaces, not implement.

C. Class Demo must implement methodY() from interface Y.

D. No error, code is correct.

Solution

1. Step 1: Check implemented interfaces and methods

   Class Demo implements interfaces X and Y, so it must define all methods from both.

2. Step 2: Identify missing method

   methodY() from interface Y is not implemented, causing an error.

3. Final Answer:

   Class Demo must implement methodY() from interface Y. -> Option C

4. Quick Check:

   All interface methods must be implemented [OK]

Hint: Implement all methods from every interface you list [OK]

Common Mistakes:

• Implementing only some interface methods
• Confusing extends with implements
• Assuming interfaces can be partially implemented

5. Given these interfaces and classes:

interface Logger { public function log(string $msg); }
interface Notifier { public function notify(string $msg); }
class System implements Logger, Notifier {
  private array $logs = [];
  public function log(string $msg) { $this->logs[] = $msg; }
  public function notify(string $msg) { echo "Notify: $msg\n"; }
  public function getLogs(): array { return $this->logs; }
}
$sys = new System();
$sys->log('Start');
$sys->notify('Alert');
$sys->log('End');
print_r($sys->getLogs());

What is the output of this code?

hard

A. Notify: Alert
Array
(
[0] => Start
[1] => End
)

B. Array
(
[0] => Start
[1] => End
)
Notify: Alert

C. Notify: Alert
Array
(
[0] => Alert
)

D. Syntax error due to missing interface methods

Solution

1. Step 1: Trace method calls

   First, log('Start') adds 'Start' to logs array. Then notify('Alert') echoes 'Notify: Alert'. Then log('End') adds 'End' to logs.

2. Step 2: Understand print_r output

   print_r outputs the logs array with 'Start' and 'End' entries after the echoed notify message.

3. Final Answer:

   Notify: Alert Array ( [0] => Start [1] => End ) -> Option A

4. Quick Check:

   Echo notify first, then print logs array [OK]

Hint: Echo outputs appear before print_r array display [OK]

Common Mistakes:

• Assuming print_r output appears before echo
• Confusing array contents logged
• Expecting syntax errors without missing methods