function create_post_for_new_user($user_id){
$user = get_userdata($user_id);
$username = $user->user_login;
 
// Post data
$post_data = array(
'post_author' => $user_id,
'post_title' => $username . "'s collection",
'post_status' => 'publish',
'post_type' => 'collection'
);
 
// Create the new post
wp_insert_post($post_data);
}
 
// Hook into 'user_register'
add_action('user_register', 'create_post_for_new_user');