Do you think i can lower my overall RAM usage if i only include the classes and functions on the pages i need the class/functions on?
For example, if i need the class "api.php" only on useapi.php, would i save/conserve memory by including it only on that page instead of globally in my header? Please let me know of the answer.
You sure would. The fewer symbols you need to use, the less memory it will use to run. This is why I've never recommended a single script to handle *every* custom function in use, rather split the scripts into logical groups of *what* the functions do. Classes are simply an extension on this same logic. Single script per class chained with namespace autoloading means I can limit the memory in use to just the required classes. Only downside is I sacrifice a bit of time since I don't explicitly issue an include/require and let the namespace resolution do its job instead. The time wasted is completely negligible, and IMO is well worth the flexibility by never issuing a require/include for my classes.
__________________
As of PHP 5.5, the MySQL library has been officially deprecated. It is recommended to move to either MySQLi or PDO libraries for your mysql connectivity. See here for help choosing which interface you prefer: http://php.net/manual/en/mysqlinfo.api.choosing.php