Register a function with the spl provided __autoload stack. If the stack is not yet activated it will be activated. If no parameter is provided the default implementation spl_autoload will be registered. When registering is successful the return value is true and upon failure false is being returned.
If your code has an existing __autoload function then this function must explicitly registered on the __autoload stack. This is because spl_autoload_register() will effectively replace the engine cache for the __autoload function by either spl_autoload() or spl_autoload_call().