php - "Call to a member function prepare()"背后的逻辑

Question

我正在编写一个函数来在执行任何操作之前检查数据库中的记录，但我收到错误 Call to a member function prepare()我不太明白。我已经挣扎了很长一段时间了，我真的很感谢一些帮助

问题应该出在 prepare() 上在第 19 行

<?php
$dsn = "xxx"; // Database Source Name
$username="xxx"; // User with acress to database. Root is MySQL admin.
$password="xxx"; //The user password.

try {
  $conn = new PDO($dsn, $username, $password);
  $conn ->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e) {
  echo "Connection failed: ".$e->getMessage();
}

//------------------------ Does the category already exist? -------------------------
function checkuser($fbid,$fbfname,$fblname,$femail) {

$sql="SELECT COUNT(*) AS subjectcount FROM Users WHERE Fuid=:Fuid";

try {
    $stmt = $conn->prepare($sql);
    $stmt->bindValue(":Fuid",$_SESSION['FBID'], PDO::PARAM_INT);
    $stmt->execute();
    $row = $stmt->fetch(PDO::FETCH_ASSOC);
    $subjectcount=$row["subjectcount"];
    } catch (PDOException $e) {
        echo "Server Error - try again!".$e->getMessage();
    }

//------------------------ If it dosn't, insert it -------------------------
if ($subjectcount==0) {

而且我很难调试，因为我不太明白这个错误的原因。

我将代码更新为

<?php
$dsn = "xxx"; // Database Source Name
$username="xxx"; // User with acress to database. Root is MySQL admin.
$password="xxx"; //The user password.

try {
  $conn = new PDO($dsn, $username, $password);
  $conn ->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e) {
  echo "Connection failed: ".$e->getMessage();
}

//------------------------ Does the category already exist? -------------------------
function checkuser($fbid,$fbfname,$fblname,$femail) {
global $conn;
$sql="SELECT COUNT(*) AS subjectcount FROM Users WHERE Fuid=:Fuid";

try {
    $stmt = $conn->prepare($sql);
    $stmt->bindValue(":Fuid",$_SESSION['FBID'], PDO::PARAM_INT);
    $stmt->execute();
    $row = $stmt->fetch(PDO::FETCH_ASSOC);
    $subjectcount=$row["subjectcount"];
    } catch (PDOException $e) {
        echo "Server Error - try again!".$e->getMessage();
        exit;
    }

//------------------------ If it dosn't, insert it -------------------------
if ($subjectcount==0) {

Answer 1

我猜完整的错误消息是:

Fatal error: Call to a member function prepare() on a non-object

对吗？

您正在调用变量$conn的成员函数(=类实例成员的函数)prepare，该变量未在您的作用域中声明checkuser 函数，它是在外部声明的(在全局范围内)!

要将其“导入”到您的函数中以便您可以访问它，请将此行放在函数的顶部:

global $conn;

此外，您似乎没有启用完整的错误报告，否则您会在 fatal error 之前看到此错误，从而为您提供另一条线索:

Notice: Undefined variable: conn

(顺便说一句，在 catch block 中输出数据库错误消息后，您应该 exit; - 否则您将打印错误但继续其余部分，使用不存在的数据库连接!)