Call to a member function fetch() on a non-object - php

Question

I'm getting the error:
Call to a member function fetch() on a non-object
The line this refers to is:
$getProjectIdResult = $stmt->fetch();
Now, I think from this error that there must be something wrong with my database query, since the documentation says PDO query returns false on failure. I'm having trouble identifying what is causing the issue.
I've tried wrapping the fetch in a try/catch, with
$this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
However the catch isn't triggered and I just get the original fatal error so I haven't been able to get a more specific error.
classes.php
class Query extends Connection {
public function getProjectID($surveyID) {
$query_getProjectID = "SELECT projectID FROM test WHERE surveyID = :surveyID";
$query_getProjectID_params = array(
':surveyID' => $surveyID
);
try {
$stmt = $this->db->prepare($query_getProjectID);
$stmt = $stmt->execute($query_getProjectID_params);
}
catch (PDOException $ex) {
die("Failed to get project ID: " . $ex->getMessage());
}
$getProjectIdResult = $stmt->fetch();
$getProjectID = $getProjectIdResult['projectID'];
return $getProjectID;
}
}
test.php
include_once("includes/classes.php");
include_once("includes/functions.php");
// Bind $_GET data
// localhost/panel/test.php?surveyID=3&status=1&respondentID=666
// Expected result: 111
$surveyID = sanitise($_GET['surveyID']);
$status = sanitise($_GET['status']);
$respondentID = sanitise($_GET['respondentID']);
$con = new Connection();
$query = new Query();
$query->getProjectID($surveyID);
$con->closeConnection();
I've ruled out the sanitise function causing an issue by testing with and without it.
I apologise as I know this is probably just another amateur making another amateur mistake judging by how many posts there are by the same title.

Answer 1

When you call
$stmt = $stmt->execute($query_getProjectID_params);
You assign the return-value of execute() to $stmt, overwriting the variable, making it a boolean instead of an object. When you continue, $stmt no longer holds the PDOStatement object, but is now a boolean.
The solution is simply to remove the overwrite of your object, like this (remove $stmt = in front).
$stmt->execute($query_getProjectID_params);
http://php.net/pdostatement.execute