What do the access levels of elements in object-oriented programming mean, and what are the types?

In object-oriented programming (OOP), access levels determine which parts of a program can access certain class elements (properties and methods). In PHP, there are three main access levels:

1. public — elements are accessible from anywhere, both inside and outside the class.
2. protected — elements are accessible only within the class itself and its subclasses.
3. private — elements are accessible only within the class where they are declared and are not accessible in subclasses or outside.

Example:

class Example {
    public $publicVar = 'public';
    protected $protectedVar = 'protected';
    private $privateVar = 'private';

    public function testAccess() {
        echo $this->publicVar;    // Accessible
        echo $this->protectedVar; // Accessible
        echo $this->privateVar;   // Accessible
    }
}

class ChildExample extends Example {
    public function testAccess() {
        echo $this->publicVar;    // Accessible
        echo $this->protectedVar; // Accessible
        // echo $this->privateVar; // Error: not accessible
    }
}

$obj = new Example();
echo $obj->publicVar;      // Accessible
// echo $obj->protectedVar; // Error
// echo $obj->privateVar;   // Error

Thus, access levels help encapsulate data and control its visibility.