How to email user after inserting the username in database in WordPress

Question

I have following piece of code, which inserts usernames and other details of users in the database. After inserting the usernames I want to email them using wp_mail();. I am unable to do so. How can I do this?

$member_details->user_login = array_map( 'sanitize_text_field', $_POST['user_login'] );
$member_details->user_role = array_map( 'sanitize_text_field', $_POST['user_role'] );
$member_details->status = array_map( 'sanitize_text_field', $_POST['status'] );

$member_details_encode = wp_json_encode( $member_details );

  global $wpdb;

  $member_result = $wpdb->insert( 'wpxa_project_members',

         array( 

              'project_id'     => $_SESSION['project_id'],
              'author_id'      => $post_author,
              'member_details' => $member_details_encode

              ),

         array( 

              '%d',
              '%d',
              '%s'

      )

    );

Answer 1

Assuming that $member_details->user_login is already a user in the wp_users table, then you could use the following to get their email address:

$user = get_user_by( $member_details->user_login, 'login' );

From there, you have the user's email address and can use wp_mail() to email them:

$subject = "My email subject";
$message = "My message body...";
wp_mail( $user->user_email, $subject, $message );

Answer 2

Let us see what code you are trying. Assuming that the hook you have bound that function to is firing, you would insert this into your function, assuming you want to email the author.

wp_mail($to = $post_author, $subject = 'This is the subject', $message = 'This is the message');

https://developer.wordpress.org/reference/functions/wp_mail/

Edit:

$user = get_userdatabylogin($member_details->user_login);
$user_id = $user->ID;
wp_mail($user_id, 'This is the subject', 'This is the message');