PDO INSERT query 500 server error [duplicate] - php

Question

This question already has answers here:
Why does this PDO statement silently fail?
(2 answers)
PHP's white screen of death [duplicate]
Closed 5 years ago.
I am new to using PDO and I am getting an http 500 server error when the form is submitted.
The php page with the processing code is in the correct folder so I don't know why its throwing up a 500 error.
There is NO url rewrite going on neither .
Here is my code:
try {
$dbh = new PDO("mysql:host=$hostname;dbname=crm",$username,$password);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); // <== add this line
$stmt = $db->prepare("INSERT INTO testdrives (forename, surname,phone,email,add1,add2,add3,city,county,postcode,car,date) VALUES (:forename, :surname,:phone,:email,:add1,:add2,:add3,:city,:county,:postcode,:car,:date)");
$stmt->bindParam(':forename', $_POST['forename']);
$stmt->bindParam(':surname', $_POST['surname']);
$stmt->bindParam(':phone', $_POST['phone']);
$stmt->bindParam(':email', $_POST['email']);
$stmt->bindParam(':add1', $_POST['add1']);
$stmt->bindParam(':add2', $_POST['add2']);
$stmt->bindParam(':add3', $_POST['add3']);
$stmt->bindParam(':city', $_POST['city']);
$stmt->bindParam(':county', $_POST['county']);
$stmt->bindParam(':postcode', $_POST['postcode']);
$stmt->bindParam(':car', $_POST['car']);
$stmt->bindParam(':date', $_POST['date']);
$stmt->execute();
if ($dbh->query($sql)) {
echo "<script type= 'text/javascript'>alert('New Record Inserted Successfully');</script>";
}
else{
echo "<script type= 'text/javascript'>alert('Data not successfully Inserted.');</script>";
}
$dbh = null;
}
catch(PDOException $e)
{
echo $e->getMessage();
}

Answer 1

Edit:
I missed the incorrect variable used for the connection being $dbh and $db in the prepare; my bad.
Original answer.
This line:
if ($dbh->query($sql)) {...}
is failing for two reasons:
Calling query() on what is already being prepared/executed.
Using a non-existant variable, $sql.
Get rid of that statement with the related brace and replace it with simply and replacing the $stmt->execute(); with:
if($stmt->execute()){
// success
} else{
// error
}
and using PDO's error handling (as you are doing now) and PHP's error reporting:
http://php.net/manual/en/pdo.error-handling.php
http://php.net/manual/en/function.error-reporting.php
Check your logs also.

Answer 2

Found the problem
The above code shows
$stmt = $db->prepare
Needed to be changed to
$stmt = $dbh->prepare
Thanks for help with the other issue
Quick question how to i insert into a table that has an auto increment column ?