mysql_query() returning boolean and causing mysql_fetch_assoc() to fail

I'm writing a small login class for an application of mine, however, I think my query is bad, because when I call mysql_fetch_assoc() on the result of the query, I get this error:

Warning: mysql_fetch_assoc() expects parameter 1 to be resource, boolean given

I'm familiar with how mysql_fetch_assoc() works, but I'm guessing the call to mysql_query() is returning false, which is obviously a boolean, producing the error.

Here's the query:

$loginsql = 'SELECT userid, username, password FROM users WHERE username=\'. $username .\' AND password=\'. $password .\'';

Note: I realize the "mysql_" function set in PHP is deprecated as of 5.5, but I'm using 5.3.8 and just practicing. I will refactor the application later using PDO.

classes.php

<?php

class connectToDb {
    function dbConnect($config) {   
        $connection = mysql_connect($config['host'], $config['dbuser'], $config['dbpass']);
        if ($connection) {
            mysql_select_db($config['db'], $connection);
        } else {
            echo "Could not connect to database!";
        }
    }
}

class registerAccount {
    function doRegister($regusername, $regpassword, $regemail) {
        $regsql = "INSERT INTO users (username, password, email) VALUES ('$regusername', '$regpassword', '$regemail')";
        if (mysql_query($regsql)) {
            echo "Successfully registered!";
        } else {
            echo "Problem with registration!";
        }
    }
}

class loginAccount {
    function doLogin($username, $password) {
        mysql_real_escape_string($username);
        mysql_real_escape_string($password);

        hash('sha256', $password);

        $loginsql = 'SELECT userid, username, password FROM users WHERE username=\'. $username .\' AND password=\'. $password .\'';

        $result = mysql_query($loginsql) or die(mysql_error());

        $loginrow = mysql_fetch_assoc($result);
        if ($loginrow) {
            $_SESSION['username'] = $loginrow['username'];
            $_SESSION['userid'] = $loginrow['userid'];
        } else {
            echo "Incorrect username and/or password!";
        }
    }
}