导致登录错误一个Wordpress的原因

Question

请查看以下代码，这是针对我的网站前端通过wordpress而没有使用wordpress平台，但登录页面有问题。

<?php
    global $wpdb;

    $err = '';
    $success = '';

    if(isset($_POST['task']) && $_POST['task'] == 'login' )
    {
        //We shall SQL escape all inputs to avoid sql injection.
        $username = $wpdb->escape($_POST['log']);
        $password = $wpdb->escape($_POST['pwd']);
        $remember = $wpdb->escape($_POST['remember']);


        if( $username == "" || $password == "" ) {
            $err = 'Please don\'t leave the required field.';
        } else {
            $user_data = array();
            $user_data['user_login'] = $username;
            $user_data['user_password'] = $password;
            $user_data['remember'] = $remember;  
            $user = wp_signon( $user_data, false );

            if ( is_wp_error($user) ) {
                $err = $user->get_error_message();
                exit();
            } else {
                wp_set_current_user( $user->ID, $username );
                do_action('set_current_user');

                echo '<script type="text/javascript">window.location='. get_bloginfo('url') .'</script>';  
                exit(); 
            }
        }
    }
?>

这是用于登录的表格。

<form method="post">
    <div class="message"><h2>Already have an account? Please login.</h2></div>
    <p>
    <?php 
        if( !empty($sucess) ) 
            echo $sucess;

        if( !empty($err) )
            echo $err;
        ?>
    </p>
        <input type="text" name="log" value="" id="log" class="textbox" placeholder="Username"/>
        <input type="password" name="pwd" value="" id="pwd" class="textbox" placeholder="Password"/>
    <p><input type="submit" value="Login" class="button" />

    <label><input type="checkbox" name="remember" value="true" /> Remember Me</label>
    <input type="hidden" name="task" value="login" />
</form>

为什么我会收到以下信息：警告无法修改标题信息

我如何解决这个问题？