What is the difference between public, private, and protected in PHP?

<?php

// BaseClass
class pub {
    public $tag_line = "A Computer Science Portal for Geeks!";
    function display() {
        echo $this->tag_line."<br/>";
    }
}

// SubClass
class child extends pub {
    function show(){
        echo $this->tag_line;
    }
} 

// Object Declaration
$obj= new child;

// A Computer Science Portal for Geeks!
echo $obj->tag_line."<br/>"; 

// A Computer Science Portal for Geeks!
$obj->display(); 

// A Computer Science Portal for Geeks!
$obj->show(); 
?>

<?php

// Base Class
class pro {
    protected $x = 500;
    protected $y = 500;
            
    // Subtraction Function
    function sub() 
    {
        echo $sum=$this->x-$this->y . "<br/>";
    }     
} 

// SubClass - Inherited Class
class child extends pro {
    function mul() //Multiply Function
    {
        echo $sub=$this->x*$this->y;
    }
} 

$obj= new child;
$obj->sub();
$obj->mul();
?>

<?php

// Base Class
class demo {
    private $name="A Computer Science Portal for Geeks!";
    
    private function show()
    {
        echo "This is private method of base class";
    }
} 

// Sub Class
class child extends demo {
    function display()
    {
        echo $this->name;
    }
} 

// Object Declaration
$obj= new child;

// Uncaught Error: Call to private method demo::show()
$obj->show(); 

//Undefined property: child::$name
$obj->display(); 
?>

<?php
class BaseClass
{
    public $public = 'Public';
    protected $protected = 'Protected';
    private $private = 'Private';

    function Display()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj = new BaseClass();
echo $obj->public; 
echo $obj->protected; // Cannot access protected property
echo $obj->private; // Cannot access private property
$obj->Display();  //Displays all properties

class SubClass extends BaseClass
{
    public $public = 'Public Sub Class';
    protected $protected = 'Protected Sub Class';

    function Display()
    {
        echo $this->public;
        echo $this->protected;
        echo $this->private;
    }
}

$obj2 = new SubClass();
echo $obj2->public; 
echo $obj2->protected; // Cannot access protected property
echo $obj2->private;  // Cannot access private property
$obj2->Display(); //Displays all properties
?>