“调用成员函数prepare（）”背后的逻辑[重复]

I'm writing a function to check for records in my database before executing anything, and i'm getting the error Call to a member function prepare() which i don't quite understand. I've been struggeling for quite some time now, and i'd really appriciate some help

The problem should be with the prepare() in line 19

<?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) {

And i'm having a hard time debugging since i dont quite understand the cause of this error.

I updated my code to

<?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) {

</div>

I guess the full error message is:

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

Right?

You are calling the member function (= function which is a member of a class instance) prepare of the variable $conn which is not declared in the scope of your checkuser function, it's declared outside (in global scope)!

To "import" it into your function so you can access it, put this line at the top of your function:

global $conn;

Also, it looks like you don't have full error reporting enabled, otherwise you would have seen this error before the fatal one, giving you another clue:

Notice: Undefined variable: conn

(By the way, you should exit; after outputing the DB error message in your catch block - otherwise you will print the error but continue to the rest, with a nonexisting DB connection!)