PHP PDO statement throwing fatal error

I have a function which is throwing an unusual error regarding a syntax issue. Have a look.

public static function authenticate($_user, $_pass)
    {
        $sql = 'SELECT password, key 
                FROM users 
                WHERE username = ' . $_user;

        $stm = Db::init()->prepare($sql);
        if ($stm->execute())
            return $stm->fetch(PDO::FETCH_ASSOC);    
    }

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 开发者_Go百科use near 'key FROM users WHERE username = testuser1' at line 1' in /class.php:111

Stack trace:

#0 /class.php(111): PDOStatement->execute()

#1 /class.php(118): Password::authenticate('testuser1', 'test')

#2 {main} thrown in /class.php on line 111

Any ideas as to what this means?

key is a reserved word in sql. Surround it with backticks in your query. Like this:

public static function authenticate($_user, $_pass)
    {
        $sql = 'SELECT password, `key` 
                FROM users 
                WHERE username = ' . $_user;

        $stm = Db::init()->prepare($sql);
        if ($stm->execute())
            return $stm->fetch(PDO::FETCH_ASSOC);    
    }

BTW: You have a SQL injection vulnerability in your code. Use a parameterized query to bind the value of $_user.