Php pdo中的参数错误

我有一个php文件，作为程序和数据库之间的抽象。我试图让它返回一个值，但出现以下错误：

ERROR: 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 ': USERNAME
        ORDER BY id DESC
        LIMIT 1' at line 2

我的API的全部代码如下：

<?php
header("Content-Type: application/json; charset=UTF-8");
require 'UHCauth.php';
try {
    $conn = new PDO("mysql:host=$mysql_serv;dbname=$mysql_db", 
$mysql_user, $mysql_pass);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

if(isset($_GET['USERNAME'])) {
    $USERNAME = $_GET['USERNAME'];
    $stmt = $conn->prepare(
        "SELECT * FROM $mysql_table
        WHERE USERNAME = : USERNAME
        ORDER BY id DESC
        LIMIT 1"
    );
    $stmt->execute(array('USERNAME' => $USERNAME));

    $row = $stmt->fetch(PDO::FETCH_ASSOC);

    echo json_encode($row);
} else {
    $stmt = $conn->prepare(
        "SELECT * FROM $mysql_table
        ORDER BY
            McVersion DESC,
            ModVersion DESC
        LIMIT 1"
    );
    $stmt->execute();

    $row = $stmt->fetch(PDO::FETCH_ASSOC);

    echo json_encode($row);
}
} catch(PDOException $e) {
    echo 'ERROR: ' . $e->getMessage();
}
?>

它应该获取一个用户名，并以JSON格式返回该用户名的行。此代码以前适用于其他项目，现在决定不使用。

更改此代码

 WHERE USERNAME = : USERNAME
                   ^--//dont make sapce here

到

---

是的。这解决了我的问题。不知道那空间是怎么潜入的。

 WHERE USERNAME = :USERNAME