How to update a custom field in all posts with the value of another custom field in the same post?

$enddatevar = get_post_meta( $post_id, $key = 'enddate', $single = false);

$post_id is not defined, either declare it beforehand, or use $post->ID.

update_post_meta( $post->ID, 'promote', '$enddatevar' );

‘$enddatevar’ shouldnt be inside quotes.