Motivations for Current Design

As of version 4.6, Drupal does not use PHP's class construct. This decision was made for several reasons.

First, PHP's support for object-oriented constructs was much less mature at the time of Drupal's design. Drupal was built on PHP 4, and most of the improvements in PHP 5 relate to its object-oriented features.

Second, Drupal code is highly compartmentalized into modules, each of which defines its own set of functions. The inclusion of files is handled inside functions as well; PHP's performance suffers if all code is included on each page call, so Drupal attempts to load as little code as possible per request. This is a critical consideration, especially in the absence of a PHP accelerator; the act of compiling the code accounts for more than half of a Drupal page request. Functions are therefore defined inside other functions in Drupal, with respect to the runtime scope. This is perfectly legal. However, PHP does not allow the same kind of nesting with class declarations. This means that the inclusion of files defining classes must be "top-level," and not inside any function, which leads either to slower code (always including the files defining classes) or a large amount of logic in the main index.php file.

Finally, using classes to implement Drupal constructs is difficult due to the use of some advanced object-oriented design patterns used by Drupal itself. While this may sound self-contradictory, it should become clear in the following discussion that the lack of certain OOP constructs such as Objective-C's "categories" in PHP would mean that implementing some Drupal mechanisms (such as the theme system) would be more complicated using classes than using functions.

source:http://api.drupal.org/?q=api/file/developer/topics/oop.html/4.7