Constants in classes in PHP - Step-by-Step Execution

Concept Flow - Constants in classes

Define class with constant

↓

Access constant via ClassName::CONSTANT

↓

Use constant value in code

↓

Output or use constant value

↓

End

This flow shows how a class constant is defined, accessed using ClassName::CONSTANT, and then used in the program.

Execution Sample

PHP

<?php
class Car {
  const WHEELS = 4;
}
echo Car::WHEELS;
?>

Defines a class Car with a constant WHEELS and prints its value.

Execution Table

Step	Action	Code Line	Evaluation	Output
1	Define class Car with constant WHEELS=4	class Car { const WHEELS = 4; }	Class Car created with constant WHEELS=4
2	Access constant WHEELS from Car	Car::WHEELS	Value 4 retrieved
3	Print constant value	echo Car::WHEELS;	Output 4	4
4	End of script

💡 Script ends after printing the constant value 4.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Car::WHEELS	undefined	4	4	4	4

Key Moments - 2 Insights

Why do we use ClassName::CONSTANT instead of $this->CONSTANT?

Class constants are accessed with ClassName::CONSTANT because they belong to the class itself, not to an instance. This is shown in execution_table step 2 where Car::WHEELS is used.

Can we change the value of a class constant after defining it?

No, class constants cannot be changed after definition. The execution_table shows the constant value stays 4 throughout.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of Car::WHEELS at step 2?

A null

B undefined

C 4

D 0

💡 Hint

Check the 'Evaluation' column at step 2 in the execution_table.

At which step does the program output the constant value?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the 'Output' column in the execution_table.

If we tried to change Car::WHEELS after definition, what would happen?

A The value changes successfully

B The value remains unchanged

C A syntax error occurs

D The program outputs the new value

💡 Hint

Refer to key_moments about immutability of class constants.

Concept Snapshot

Class constants are fixed values inside a class.
Define with 'const NAME = value;'.
Access with ClassName::NAME.
Cannot be changed after definition.
Useful for fixed data related to the class.

Full Transcript

This example shows how to define and use constants inside PHP classes. We define a class Car with a constant WHEELS set to 4. We access this constant using Car::WHEELS and print its value. Class constants belong to the class itself, not instances, so we use :: to access them. Constants cannot be changed after they are set. The execution table traces each step from defining the class, accessing the constant, printing it, and ending the script. Key moments clarify why we use ClassName::CONSTANT and that constants are immutable. The quiz tests understanding of these points.