Access modifiers (public, private, protected) in PHP - Step-by-Step Execution

Concept Flow - Access modifiers (public, private, protected)

Define Class with Members

↓

Access Modifier Check

↓

Public

↓

Accessible Anywhere

↓

Access Attempt

↓

Allowed or Error

This flow shows how PHP checks access modifiers when you try to use class members: public is open everywhere, protected is limited to class and subclasses, private only inside the class.

Execution Sample

PHP

<?php
class Box {
  public $width = 10;
  private $height = 20;
  protected $depth = 30;
}
$box = new Box();
echo $box->width;
echo $box->height; // error
?>

This code defines a class with public, private, and protected properties and tries to access them from outside the class.

Execution Table

Step	Action	Member Access	Access Modifier	Allowed?	Output / Result
1	Create object $box	-	-	Yes	Object created
2	Access $box->width	width	public	Yes	10
3	Access $box->height	height	private	No	Fatal error: Cannot access private property Box::$height
4	Access $box->depth	depth	protected	No	Fatal error: Cannot access protected property Box::$depth

💡 Execution stops at step 3 due to fatal error accessing private property from outside class.

Variable Tracker

Variable	Start	After Object Creation	Final
$box->width	undefined	10	10
$box->height	undefined	20	20
$box->depth	undefined	30	30

Key Moments - 2 Insights

Why can I access the public property but get an error with private or protected?

Public properties are open to all code (see step 2 in execution_table). Private and protected restrict access: private only inside the class (step 3 error), protected inside class and subclasses (step 4 error).

What is the difference between private and protected?

Private means only the class itself can access the member. Protected allows access inside the class and any subclass. Both block outside access (see steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what happens when accessing $box->depth from outside the class?

A It outputs nothing

B It outputs 30

C It causes a fatal error

D It returns null

💡 Hint

Check step 4 in the execution_table where $box->depth is accessed.

At which step does the program stop due to an error?

A Step 3

B Step 2

C Step 4

D After all steps

💡 Hint

Look at the exit_note and step 3 in execution_table.

If $height was declared public instead of private, what would change in the execution_table?

A Step 4 would cause an error instead

B Step 3 would allow access and output 20

C Step 3 would still cause an error

D No change at all

💡 Hint

Think about how public access works from the concept_flow and execution_table step 2.

Concept Snapshot

Access modifiers control visibility of class members:
- public: accessible anywhere
- protected: accessible in class and subclasses
- private: accessible only inside the class
Trying to access private or protected from outside causes errors.
Use modifiers to protect data and control access.

Full Transcript

This visual execution shows how PHP access modifiers work. We define a class with public, private, and protected properties. When we create an object and try to access these properties from outside, public works fine, but private and protected cause errors. Public means open to all code. Private means only the class itself can access. Protected means the class and its subclasses can access. Trying to access private or protected from outside the class causes fatal errors. This helps protect data inside objects and control how other code interacts with them.