I know in the Manuel it says that the header has to be the first thing in a script, but how come I see some codes where header("Location: member.php?id=$username") is in a if-statement?

Ex:

//a bunch of codes above

if($result!="0"){ 
// authenication correct lets login
$_SESSION["password"] = $password;;
$_SESSION["username"] = $username;
header("Location: member.php?id=$username");
}
else 
{
    echo "Wrong username or password. Please try again!";
}

But when I do this, it sometimes would/won't throw an error. How do I allow the header (); to be used in a script without any errors? I want to redirect the user back to the login if they click "no" and to the homepage if they click "yes".