pp-wp-snippet-force-email-login.php

<?php
// This code snippet forces login with email address and eliminates login with user name

// Remove the default authentication function and replace it by a custom function
remove_filter( 'authenticate', 'wp_authenticate_username_password', 20, 3 );
add_filter( 'authenticate', 'force_email_login', 20, 3 );

// Custom function to force email login
function force_email_login( $user, $username, $password ) {

  if ( ! empty( $username ) ) {
    
    // Is there a user with the given email address?
    $user = get_user_by( 'email', str_replace( '&', '&amp;', $username ) );

    if ( ! $user ) {
      // No user with the given mailaddress was found, so we set the username to a silly string, that will always result
      // in an invalid username error - so we don't have to write our own code and we don't give any hints, because
      // the default WordPress error message will be shown
      $username = 'THIS_IS_NOT_A_VALID_USERNAME_AND_LEADS_TO_AN_ERROR_WHEN_TRYING_TO_LOG_IN_WITH_THIS_SILLY_STRING';
    } else {
      // We found an user with the given mailaddress, so we set the username to the user's name
      $username = $user->user_login;
    }
  }

  // Now we call the default authentication function and let WordPress do the rest
  return wp_authenticate_username_password( null, $username, $password );

}
?>