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The wp_login action hook is triggered when a user logs in by the wp_signon() function. It is the very last action taken in the function, immediately following the wp_set_auth_cookie() call.

This hook provides access to two parameters: $user->user_login (string) and $user ( WP_User ). To pass them into your function you will need to add a priority (default is 10) and request 2 arguments from the add_action() call:

<?php
function your_function( $user_login, $user ) {
    // your code
}
add_action('wp_login', 'your_function', 10, 2);
?>