[CodeBinary]

Heres what I got:

Code

<?php
    require("inc/connection.php");
    if ($link->connect_error)
        die('Connect Error: '.$link->connect_errno.': '.$link->connect_error);
    $insertedcode = $_POST['code'];
    //$results = $link->query("SELECT code FROM code WHERE code = :code");
    $query_string = "SELECT code FROM code WHERE code = :code";
    $query_params = array(
        ':code' => $_POST['code']
    );

    $stmt = $link->prepare($query_string);
    //$stmt->bind_param('s', $query_params[':code']);
    $result = $stmt->execute($query_params);
    $row = $stmt->fetch();
    if($row) {
        $number = mt_rand(0,3999);
        echo $number;
    }
?>

But it doesnt work i get this error please help:

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

[RedWaters]

Since $link is supposed to return an object on prepare I guess it returns a bad result, checking out your query

Code

SELECT code FROM code WHERE code = :code

You should specify the column as code.code

[ASLR]

As per the documentation, PDO will return false or throw an error on failure.

Obviously it is failing. Set the PDO error mode to PDO::ERRMODE_EXCEPTION then catch the exception and examine it's message to find out why your query preparation is failing.