It depends on, where you are calling these functions. That is the point, where you enter the var into the function.
First, remove the global $post from your functions.php! It makes no sense there! In the functions.php you declare the functions, not call them!
I assume, you call your function in some template. So the functions.php should look like this:
function myfunction($post) {
// $post is now a copy of the $post object, it needs to be passed in the call. See below!
do_something($post->ID);
}
Or you could use the global keyword, to make the $post variable available inside the function:
function myFunction() {
global $post;
// now you can use $post from the global scope
do_something($post->ID);
}
So, you could use $post globally, using the global keyword inside the function (second example), or you can pass the $post variable as a function parameter (first example). Both have their pros and cons, e.g. if you pass it as a parameter, PHP copies the $post object for usage inside the function, whereas using global it uses the original. I recommend reading up on PHP scopes. 🙂
Now you have a correct declaration of the function – you are ready to call it! In your template, or whereever you are, do this:
<?php myFunction($post); ?>
or
<?php myFunction(); ?>
depending on which version you decided on. In any case, you’ll probably need to do this inside the_loop!