Case sensitive username #hook #filer

case_sensitive_username.php

<?php

function login_case_sensitive( $user, $username, $password ) {
    $user_id = username_exists($username);
    if( !$user_id ) {
        return $user;
    }
    $user_obj = get_user_by('id', $user_id);
    if( $username !== $user_obj->user_login ) {
        return new WP_Error( 'authentication_failed', __( '<strong>ERROR</strong>: Invalid username or password.' ) );
    }
    return $user;
}
add_filter( 'authenticate', 'login_case_sensitive', 20, 3 );