What are some key differences in functionality between PHP 4 and PHP 5 that could impact development?

One key difference in functionality between PHP 4 and PHP 5 is the introduction of new object-oriented features in PHP 5, such as visibility keywords (public, private, protected) and magic methods (__construct, __destruct, __get, __set). These features allow for better encapsulation and reusability of code in object-oriented programming.

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

    function MyClass() {
        $this->myVar = 'Hello';
    }

    function getMyVar() {
        return $this->myVar;
    }
}

// PHP 5 style class definition
class MyClass {
    private $myVar;

    public function __construct() {
        $this->myVar = 'Hello';
    }

    public function getMyVar() {
        return $this->myVar;
    }
}