There are three type of visibility scope:

1. Public
2. Private
3. Protected

PUBLIC:

Variable or function of public scope can be accessed throughout the program.

PRIVATE :

Variable or function of private scope can be accessed only with in the class they are declared.

PROTECTED:

Variable or function of protected scope can be accessed only by its subclass and derived class.

<?php
/**
 * Define ClassA
 */
class ClassA
{
    public $a = 1;
    protected $b = 2;
    private $c = 3;

    function detail()
    {
        echo $this->a;
        echo $this->b;
        echo $this->c;
    }
}

$obj = new ClassA();
echo $obj->a; // Works
echo $obj->b; // Fatal Error
echo $obj->c; // Fatal Error
$obj->detail(); // Shows Public, Protected and Private


/**
 * Define ClassB
 */
class ClassB extends ClassA
{
    // We can redeclare the public and protected method, but not private
    protected $b = '4';

    function detail()
    {
        echo $this->a;
        echo $this->b;
        echo $this->c;
    }
}

$obj2 = new ClassB();
echo $obj2->a; // Works
echo $obj2->b; // Fatal Error
echo $obj2->c; // Undefined
$obj2->detail(); // Shows Public, Protected2, Undefined

?>