Trick the database into accessing data

I have found this problem in my teacher s notes. I should find a way to trick the database so that the checkUsernamePassword(...) function returns TRUE.

For the beginning, I have tried to simulate this problem and call the function, but I do not understand how the first param should look(I am new to databases).

checkUserPass("users.sql",",Admin","mark56"); //this throws me an error, users.sql being exported from my olready created db in phpmyadmin

function SQLFilter($username, $password){
        $commands = array("AND", "OR", "DROP", "CREATE", "SELECT", "DELETE", "LIKE", 
        "LIMIT", "ORDER BY", "JOIN", "UNION", "INSERT", "UPDATE", "REGEXP", "WHERE", 
        "HAVING", "TRUNCATE", "DISTINCT");

    foreach($commands as $command)
    {
        $username = str_ireplace($command, "", $username);
        $password = str_ireplace($command, "", $password);
    }

    return array($username, $password);
}

function checkUsernamePassword($db, $username, $password)
{
    if (!$db) {
        echo "Error connecting to database.";
        return false;
    }

    list($username, $password) = SQLFilter($username, $password);

//It is all clear until HERE

//I've tried to verify how this function work, but I cannot realise how should I call the function checkUsernamePassword (first parameter??)

    $res = sqlite_query($db, "SELECT 1 FROM users WHERE username='$username' AND password='$password'", $error);

    if (!$res) {
        echo htmlentities($error, ENT_QUOTES); 
        return false;
    }

    $num_rows = sqlite_num_rows($res);
    sqlite_close($db);

    if ($num_rows==1)
        return true;

    return false;
}