Customer asks if a specific carousel plugin he uses can be widgetized. That means I should create a widget inside functions.php which calls the plugin’s function. That means that the plugin’s code has to be loaded first so that the function be available to WordPress when the functions.php file is loaded, right? Would that work?
The plugins are loaded right before theme (yes, I’ve been looking for excuse to use this):
However it is wrong to think about either as point of code execution. For most cases everything should be hooked and executed no earlier than init hook. According to Codex widget registration with register_widget() should be hooked to widget_init.
Because of that order of load doesn’t matter for this case, you will have everything loaded by the time widget needs it in any case.