You can create for each Event a custom post meta (example event_users) where you will save the users ID’s that clicks the “I’m interested in this event”
Something like:
$event_users = get_post_meta($event_id, 'event_users', true); // Get exisitig event users
$event_users[$user_id] = 1; // Add user ID as a KEY, a simple way to make sure you only add it once
add_post_meta($event_id, 'event_users', $event_users, true) or update_post_meta($event_id, 'event_users', $event_users); // Add/update event users
Then when you wnat to use this data to get all users for one event is easy just use the $event_users = get_post_meta($event_id, 'event_users', true);
Getting all events for a user is a bit more tricky. You can use the WP_Query with meta_query like:
[
`key` => 'event_users',
`value` => '%:'. $user_id .';%',
'compare': 'LIKE'
]
PS: Array value are serialized on saved, but to be honest I do not like to much serialized values (is just a personal thing), I prefer to convert the values to JSON (json_encode on save, and json_decode when you extract data from DB)
In case you will use json data format meta query will look something like:
[
`key` => 'event_users',
`value` => '%"'. $user_id .'"%',
'compare': 'LIKE'
]
Check this links for more info:
– https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters
– https://codex.wordpress.org/Function_Reference/add_post_meta
A dedicated plugin for events: https://wordpress.org/plugins/the-events-calendar/