PHP相同值不同的结果

Question

我遇到了一个奇怪的问题而且我真的不知道是什么导致它我有一个类来更新用户活动日志类的东西。它看起来像这样：

class activiteit
{
    private $dbc;
    public $uid;

    public function __construct(){
        require 'dbc.php';
        $this->dbc = $dbc;
        $this->uid = $this->getUserId();
        var_dump($this->uid);
    }

    //bunch of functions here....

    public function getUserId(){
        $usr = mysqli_real_escape_string($this->dbc, $_SESSION['usr']);
        $sql = "SELECT id FROM tbl_users WHERE gebruikersnaam='$usr'";

        $res = mysqli_query($this->dbc, $sql); 
        $res = mysqli_fetch_assoc($res);
        return $res['id'];
    }
}

var dump的值是这个字符串（1）＆＃34; 7＆＃34; 其余的代码不起作用。 但如果我将代码更改为此

class activiteit
{
    private $dbc;
    public $uid;

    public function __construct(){
        require 'dbc.php';
        $this->dbc = $dbc;
        $this->uid = $this->getUserId();
        var_dump($this->uid);
    }

    //bunch of functions here....

    public function getUserId(){
        $usr = mysqli_real_escape_string($this->dbc, $_SESSION['usr']);
        $sql = "SELECT id FROM tbl_users WHERE gebruikersnaam='$usr'";

        $res = mysqli_query($this->dbc, $sql); 
        $res = mysqli_fetch_assoc($res);
        return "7";
    }
}

var dump的值仍然是string（1）＆＃34; 7＆＃34; 其余的代码现在运行正常。