PDO SQLSTATE[42000] on simple select query - php

Question

Im just new with programming in OOP, so im writing a function but it gives an error, i think im using PDO wrong, actually i now it for sure, but i dont now how to fix it. This is my code im using currently:
public function takedrugs($soort, $hoeveelheid, $id){
$conn = $this->conn;
$drugsophalen = $conn->prepare('SELECT * FROM gebruikers WHERE id=:id');
$drugsophalen->execute(array(':id' => $id));
$result = $drugsophalen->fetch();
$huidigdrugs = $result[$soort];
if($huidigdrugs >= $hoeveelheid){
//Voldoende drugs dus drugs afnemen
$drugsafnemen = $conn->prepare('UPDATE gebruikers
SET :soort = :soort - :hoeveelheid,
WHERE id = :id');
$drugsafnemen->execute(array(
':soort' => $soort,
':hoeveelheid' => $hoeveelheid,
':id' => $id));
} else {
return false;
}
}
So when i use this function i get an error, its all about the SET :soort = :soort - :hoeveelheid.
This is the error i get:
Fatal error: Uncaught exception 'PDOException' with message
'SQLSTATE[42000]: Syntax error or access violation: 1064 You have an
error in your SQL syntax; check the manual that corresponds to your
MySQL server version for the right syntax to use near ''Cannabis' =
'Cannabis' - '2000', WHERE id ' at line 2' in
I hope there are some people who now how to fix it since i dont :S
Thanks in advance!

Answer 1

You CANT bind column names SEE Complex Cases in PDO info.
Also as Jason states about lazy binding use bindParam OR bindValue
TRY
$drugsafnemen = $conn->prepare('UPDATE gebruikers
SET $soort = $soort - :hoeveelheid,
WHERE id = :id');
$drugsafnemen->bindParam(':hoeveelheid', $hoeveelheid, PDO::PARAM_INT);
$drugsafnemen->bindParam(':id', $id, PDO::PARAM_INT);
$drugsafnemen->execute();

Answer 2

You have two problems:
First, by using execute() all your values are being treated as a string. This results in the syntax error:
UPDATE gebruikers SET field = 'Cannabis' - '2000' ...
I assume this is not your intention. Instead, use bindParam() so you can define these parameters as integers.
$drugsafnemen->bindParam(':soort', $soort, PDO::PARAM_INT);
Second, you should are setting the column name dynamically (:soort). As such, it too is getting interpolated with $soort, which is probably not your intention.