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/* ------------------------------------------------------------------------- *
 * Remove WordPress default authentication function
/* ------------------------------------------------------------------------- */
remove_filter('authenticate', 'wp_authenticate_username_password', 20);

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/* ------------------------------------------------------------------------- *
 * Add custom authentication function
/* ------------------------------------------------------------------------- */
add_filter('authenticate', function($user, $email, $password){

    //Check for empty fields
    if(empty($email) || empty ($password)){
        //create new error object and add errors to it.
        $error = new WP_Error();

        if(empty($email)){ //No email
            $error->add('empty_username', __('<strong>ERROR</strong>: Email field is empty.'));
        }
        else if(!filter_var($email, FILTER_VALIDATE_EMAIL)){ //Invalid Email
            $error->add('invalid_username', __('<strong>ERROR</strong>: Email is invalid.'));
        }

        if(empty($password)){ //No password
            $error->add('empty_password', __('<strong>ERROR</strong>: Password field is empty.'));
        }

        return $error;
    }

    //Check if user exists in WordPress database
    $user = get_user_by('email', $email);

    //bad email
    if(!$user){
        $error = new WP_Error();
        $error->add('invalid', __('<strong>ERROR</strong>: Either the email or password you entered is invalid.'));
        return $error;
    }
    else{ //check password
        if(!wp_check_password($password, $user->user_pass, $user->ID)){ //bad password
            $error = new WP_Error();
            $error->add('invalid', __('<strong>ERROR</strong>: Either the email or password you entered is invalid.'));
            return $error;
        }else{
            return $user; //passed
        }
    }
}, 20, 3);

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/* ------------------------------------------------------------------------- *
 * Change text "Username" in wp-login.php to "Email"
/* ------------------------------------------------------------------------- */
add_filter('gettext', function($text){
    if(in_array($GLOBALS['pagenow'], array('wp-login.php'))){
        if('Username' == $text){
            return 'Email';
        }
    }
    return $text;
}, 20);