问题描述

我只是在尝试 PDO 并且收到此错误，致命错误:在非对象上调用成员函数 fetch()，但它不是已经在 $this->db 对象上了吗?

I am just trying out PDO and I get this error, Fatal error: Call to a member function fetch() on a non-object, but isn't it already on the $this->db object?

class shoutbox {

    private $db;

    function __construct($dbname, $username, $password, $host = "localhost" ) 
    { # db conections
        try {
            $this->db = new PDO("mysql:host=".$hostname.";dbname=".$dbname, $username, $password);
        }
        catch(PDOException $e)
        {
            echo $e->getMessage();
        }
    }

    function getShouts()
    {
        $sql_shouts = $this->db->query('SELECT shoutid, message, pmuserid, ipadress, time FROM shouts WHERE pmuserid == 0');

        return $sql_shouts->fetch(PDO::FETCH_OBJ);

    }

}

推荐答案

仔细查看 PDO::query，特别是返回值"部分:

Look carefully at the documentation for PDO::query, particularly the "Return Values" section:

PDO::query() 返回一个 PDOStatement对象，或失败时为 FALSE.

PDO::query() returns a PDOStatement object, or FALSE on failure.

重要的一点是失败时为 FALSE".FALSE 不是一个对象，所以调用 ->fetch() 是个坏消息.

The important bit is "FALSE on failure". FALSE is not an object, so calling ->fetch() is bad news.

该错误可能是由于您使用了=="比较运算符.在 SQL 中，它只是=".

The error is probably due to your use of "==" comparison operator. In SQL, it's just "=".

你应该测试 $sql_shouts 是不是假的，然后对错误做一些聪明的事情，如果有的话:

You should test that the $sql_shouts is not false, and then do something smart with the error, if there was one:

<?PHP
$sql_shouts = $this->db->query('...');
if ($sql_shouts === false){
    $errorInfo = $this->db->errorInfo();
    //log the error or take some other smart action
}
return $sql_shouts->fetch(PDO::FETCH_OBJ);

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