"Cannot modify header information" bij login

<?
if (!isset($_COOKIE['timere_user']) || !isset($_COOKIE['timere_pass'])){
	if (isset($_POST['login_user']) && isset($_POST['login_pass'])) {
		$_POST['login_user'] = addslashes($_POST['login_user']);
		$_POST['login_pass'] = addslashes($_POST['login_pass']);
		$_POST['login_user'] = htmlspecialchars($_POST['login_user']);
		$_POST['login_pass'] = htmlspecialchars($_POST['login_pass']);
		
		if ($_POST['login_user'] == "" || $_POST['login_pass'] == "") {
			login_form();
			login_error();
			}
		else {
			$login_sql = mysql_query("SELECT * FROM users WHERE username='".$_POST['login_user']."' AND password='".md5($_POST['login_pass'])."'");
			$login_num = mysql_num_rows($login_sql);
	
			if (empty($login_num)){
				login_form();
				login_error();
				}
			else {
				setcookie("timere_user", $_POST['login_user'], time() + 86400);
				setcookie("timere_pass", md5($_POST['login_pass']), time() + 86400);
				login_succes();
				}
			}
		}
	else {
		login_form();
		}
	}
else {
	$login_sql = mysql_query("SELECT * FROM users WHERE username='".$_COOKIE['timere_user']."' AND password='".$_COOKIE['timere_pass']."'");
	$login_num = mysql_num_rows($login_sql);
	
	if (empty($login_num)){
		login_form();
		login_error();
		}
	else {
		login_succes();
		}
	}

function login_form() {
?>
	<form name="contact" method="post" action="">
		<div id="gebruikersnaam">Inloggen</div>
		<input type="text" name="login_user" value="Username" onFocus="this.select()" size="50"><br />
		<input type="password" name="login_pass" value="Password" onFocus="this.select()" size="50"><br />
		<input type="submit" name="login" value="Inloggen">
	</form>
<?  
	}

function login_succes() {
	$login_sql = mysql_query("SELECT * FROM users WHERE username='".$_COOKIE['timere_user']."' AND password='".$_COOKIE['timere_pass']."'");
	$login_arr = mysql_fetch_array($login_sql);
		
	if (!empty($login_arr['avatar']) && file_exists("avatars/".$login_arr['avatar'])){
		echo '<div id="avatar_frame" style="background-image:url(/avatars/'.$login_arr['avatar'].');">&nbsp;</div>';
		}
	else {
		echo '<div id="avatar_frame" style="background-image:url(/avatars/mooh.gif);">&nbsp;</div>';
		}
	echo '<div id="profiel_tekst"><a onclick="'.setcookie("timere_user").'">Uitloggen</a></div>';
	echo '<div id="gebruikersnaam">'.$login_arr['username'].' <img src="/flags/'.$login_arr['country'].'.gif" border="0" alt="'.$login_arr['country'].'" align="absmiddle" /></div>';
	}

function login_error() { 
?> 
	<div id="profiel_tekst" style="color:#FF0000;">An error incorrupted, please try again.</div>
<? 
	}
?>

Warning: Cannot modify header information - headers already sent by (output started at /home/jeffrey/domains/beklad.nl/public_html/timere/index.php:7) in /home/jeffrey/domains/beklad.nl/public_html/timere/files/login.php on line 22
Warning: Cannot modify header information - headers already sent by (output started at /home/jeffrey/domains/beklad.nl/public_html/timere/index.php:7) in /home/jeffrey/domains/beklad.nl/public_html/timere/files/login.php on line 23
Warning: Cannot modify header information - headers already sent by (output started at /home/jeffrey/domains/beklad.nl/public_html/timere/index.php:7) in /home/jeffrey/domains/beklad.nl/public_html/timere/files/login.php on line 66