PHP：设置私有变量以返回私有方法的值不起作用

问：

我有一个私有变量 $user_url，我将其初始化为“”。
所以我在公共函数中设置了它，如下所示：
但是，它总是返回 null，即使我知道它不是。
请帮忙!!!!!!$this->user_url = $this->generate_user_url();

 private $user_url;
 private $user_url_extension = '';


public function create_user(){
$this->user_url = $this->generate_user_url();

// here i echo $this->user_url; output is null

$my_url = $this->generate_user_url();
// echoing $my_url does work

}



private function generate_user_url(){
        preg_match('/\@(.*?)\./s', $this->email, $school_identifier); 
        $first_character_first_name = mb_substr($this->first_name, 0, 1);
        $temp_user_url = $first_character_first_name.$this->last_name.'@'.$school_identifier[1].$this->user_url_extension;
        $stm = $this->pdo->prepare('SELECT user_url from hm_users WHERE user_url=:user_url');
        $stm->bindParam(':user_url', $temp_user_url);
        $stm->execute();
        $res = $stm->fetch(PDO::FETCH_ASSOC);
        echo '.....................................';
            var_dump($res);
            echo '.....................................';
            echo '.....................................';
            var_dump($temp_user_url);
            echo '.....................................';
        if(!$res){
            echo '.....................................url is being returned';
            var_dump($temp_user_url);
            echo '.....................................';
            return $temp_user_url;
        }
        else{
            $this->user_url_extension = strval(rand(1, 999));
            $this->generate_user_url();
        }


      }


//everything inside private function is as expected, temp_url is set and would apear to return as that part of code is reached.
// if any one can help, please do. Thank you in advance. 

在问这里之前，我已经尝试过到处调试