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<?php
add_action('peepso_action_group_user_join', 'hook', 10, 2);
 
/*
* Send an email to admin each time user joins a group
*/
hook($group_id, $user_id) {
    $group = new PeepSoGroup($group_id);
    $user = new PeepSoUser($user_id);
 
    $subject = "{$user->get('fullname')} just joined {$group->get('name')}";
    $message = "Hi,\n\n this email is just to let you know user {$user->get('fullname')} just joined the group {$group->get('name')}\n\nHave a nice day!";
 
    mail('admin@example.com', $subject ,$message);
}