AJAX HTTP Request issue with MySQL - php

Question

I'm a newbie in AJAX and i tried to make a simple MySQL query in AJAX but it doesn't work, the response is a 500 Internal Server error.
I searched for the answer on this forum and other websites but i've never found an answer for my problem.
Basically, on my app, the user writes a pseudo and click on 'play', and an AJAX function is called onClick, a function that make a simple MySQL query :
$.ajax({
url: "check_pseudo.php",
method: "GET",
data : { pseudo : input},
error: function() {
alert("Une erreur est survenue");
},
success: function(response) {
alert(response);
}
});
PHP / MySQL :
$pseudo = $_GET["pseudo"];
$req = $bdd->prepare('SELECT ? FROM pseudo');
$req->execute(array($pseudo));
$reponse = $req->fetch();
if ($reponse) {
echo "This pseudo does exist";
}else{
echo "Pseudo not found";
}
This code sends me 'GET http://localhost:8888/check_pseudo.php?pseudo=oianf 500 (Internal Server Error)' in the console debugger.
How could i solve this problem?
Merci :)

Answer 1

You can't do this:
$req = $bdd->prepare('SELECT ? FROM pseudo');
$req->execute(array($pseudo));
Using prepare() with a ? in it builds a query expecting additional input. You need to fill that ? with something. (check out PHP's mysqli::prepare).
You need to do something like:
$req = $bdd->prepare('SELECT ? FROM pseudo');
$req->bind_param("s", $pseudo);
$req->execute();
...but even that doesn't quite seem right. Usually we wouldn't be passing the fields to be selected inside query params. We would usually do something like this:
$.ajax({
url: "users.php",
method: "GET",
data : {user_id: input_id},
error: function() {
alert("Error! ...Something went wrong.");
},
success: function(response) {
alert(response);
}
});
Then inside users.php we would have:
$user_id = $_GET["user_id"];
$req = $bdd->prepare('SELECT * FROM users where user_id=?');
$req->bind_param("i", $user_id);
$req->execute();
$reponse = $req->fetch();
if ($reponse) {
echo "This user does exist";
} else {
echo "User not found";
}