I think it must be pretty basic question but I am only starting. Can someone have a look at the 3 versions of the same (?) code below and say what the difference is? All of them
If you're inside a WordPress loop, then $post->ID it's the same as using get_the_ID()
$post->ID
get_the_ID()
You shouldn't need to globalize $post since it's already in the scope of a WordPress loop.
$post
I've never seen code using $the_ID, so I would avoid using that.
$the_ID
The safest choice would be to use get_the_ID()