php5 coding class objects best practices

hi people, i havent used thi slist for quite a while. anyway. Im trying to write an application which relies heavily on mysql. so i have made a class object which has all the functions to make using mysql very easy for me. but the problem im faced with now is, i have a bunch of other classes, which have a dependency to be able to read or write to mysql at some point. i dont want to have to write and maintain seperate mysql classes to extend other other classes. is it alright to, for example, point to one class from within another? e.g. $user = new User(); $user->mysql = new MySql(); and then call functions in MySql() from within User() by; $this->mysql->fetch_result_into_array(); im new to PHP5 so im not aware of all its new and awesome OO features. does anyone have any advise?