Interface constants in PHP - Step-by-Step Execution

Concept Flow - Interface constants

Define Interface with Constants

↓

Implement Interface in Class

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Access Constants via Interface or Class

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Use Constant Values in Code

This flow shows how to define constants inside an interface, implement the interface in a class, and access those constants.

Execution Sample

PHP

<?php
interface MyInterface {
  const VERSION = "1.0";
}
class MyClass implements MyInterface {}
echo MyClass::VERSION;
?>

Defines an interface with a constant, implements it in a class, and prints the constant value.

Execution Table

Step	Action	Evaluation	Result
1	Define interface MyInterface with constant VERSION	const VERSION = "1.0"	Interface MyInterface created with VERSION = "1.0"
2	Define class MyClass implementing MyInterface	class MyClass implements MyInterface {}	Class MyClass created, inherits interface constants
3	Access constant VERSION via MyClass::VERSION	MyClass::VERSION	"1.0"
4	Print constant value	echo MyClass::VERSION	Output: 1.0

💡 Program ends after printing the constant value.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
MyInterface::VERSION	undefined	"1.0"	"1.0"	"1.0"	"1.0"
MyClass::VERSION	undefined	undefined	"1.0"	"1.0"	"1.0"

Key Moments - 2 Insights

Why can we access the constant using the class name even though the constant is defined in the interface?

Because when a class implements an interface, it inherits the interface's constants, so MyClass::VERSION is valid as shown in execution_table step 3.

Can interface constants be changed after definition?

No, interface constants are always public, static, and final, so their values cannot be changed after definition, as implied by the constant's fixed value in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of MyClass::VERSION?

A "VERSION"

B "1.0"

C undefined

D null

💡 Hint

Refer to execution_table row 3 where MyClass::VERSION evaluates to "1.0".

At which step does the program output the constant value?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Check execution_table row 4 where echo prints the output.

If the interface constant VERSION was changed to "2.0", what would MyClass::VERSION output?

A "1.0"

B Error

C "2.0"

D null

💡 Hint

Interface constants are inherited; changing VERSION in interface changes the value accessed by the class, see variable_tracker.

Concept Snapshot

Interface constants in PHP are defined inside interfaces using const keyword.
They are public, static, and final by default.
Classes implementing the interface inherit these constants.
Access constants via InterfaceName::CONSTANT or ClassName::CONSTANT.
Constants cannot be changed after definition.

Full Transcript

This example shows how to define constants inside a PHP interface and access them through a class that implements the interface. First, the interface MyInterface is defined with a constant VERSION set to "1.0". Then, a class MyClass implements this interface. Because interface constants are inherited, MyClass can access VERSION using MyClass::VERSION. Finally, the program prints the value "1.0". Interface constants are always public, static, and final, so their values cannot be changed after definition. This makes them useful for fixed values shared across classes.