Allow login via username or password for WordPress

allow-login-via-email.php

<?php
namespace KnowTheCode;

add_filter( 'authenticate', __NAMESPACE__ . '\enable_login_with_email', 20, 3 );
/**
 * Allow the user to log in via email address.
 *
 * If the `$email_or_username` parameter is not an email, then bail out, returning the `$user`.
 * Otherwise, get the user's object via the email.  Then reauthenticate their login
 * via `wp_authenticate_username_password`.
 *
 * @since 1.0.0
 *
 * @param WP_User|WP_Error|null $user WP_User or WP_Error object from a previous callback
 * @param string $email_or_username User's email or username from the login form.
 * @param string $password User's password
 *
 * @return mixed
 */
function enable_login_with_email( $user, $email_or_username, $password ) {
	if ( ! is_email( $email_or_username ) ) {
		return $user;
	}

	$user = get_user_by( 'email', $email_or_username );
	if ( is_wp_error( $user ) || ! $user instanceof \WP_User ) {
		return $user;
	}

	return wp_authenticate_username_password( null, $user->user_login, $password );
}