PHP OOP，未捕获错误：在字符串[duplicate]上调用成员函数setAuthor（）

I use PHP version 7 and i get always this error, when i creat a news article:

Fatal error: Uncaught Error: Call to a member function setAuthor() on string in D:\xampp\htdocs\entwicklung\tools\adminlogin\lib\mod ews ewsForm.php:14 Stack trace: #0 {main} thrown in D:\xampp\htdocs\entwicklung\tools\adminlogin\lib\mod ews ewsForm.php on line 14

Here i creat my object and would save the article in my database:

if(isset($_POST["publish"]))
{
    $news = new News();
    $title = $news->setTitle($db, $_POST["ueberschrift"]);
    $news =  $news->setNews($db, $_POST["artikel"]);
    $author = $news->setAuthor($db, $_POST["autor"]); //This is line 14

    News::newsArticleCreate($db, $title, $news, $author);

}

This is my setter-method for the author:

    function setAuthor($db, $author)
    {
        if(!empty($author))
        {
            $author = mysqli_real_escape_string($db, $author);
            return $this->author = trim($author);
        }
        else
        {
            return false;
        }
    }

What is my mistake?

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