What are the differences between PHP 4 and PHP 5 object-oriented programming syntax, and how can they impact class structure and method calls?

In PHP 4, object-oriented programming syntax was more limited and less consistent compared to PHP 5. This can impact class structure and method calls as PHP 4 did not support features like visibility keywords (public, private, protected), abstract classes, interfaces, and magic methods. To upgrade PHP 4 code to PHP 5, you need to update the syntax to adhere to PHP 5 standards.

// PHP 4 style class definition
class MyClass {
    var $property;

    function MyClass() {
        // constructor
    }

    function myMethod() {
        // method
    }
}

// PHP 5 style class definition
class MyClass {
    public $property;

    public function __construct() {
        // constructor
    }

    public function myMethod() {
        // method
    }
}