session_start有关问题

session_start问题
session_start();
echo fuck;
if ($_POST['submit']=='Register'){
header('Location:sign_in.php');
}
$db=mysql_connect('localhost','bp6am','123') or die('Unable to connect.');
mysql_select_db('web',$db) or die(mysql_error($db));
if (!isset($_SESSION['logged']) || $_SESSION['logged']!=1){
$query='select user_name from user where user_name="'.$_POST['username'].'" AND password="'.$_POST['password'].'"';
$result=mysql_query($query,$db) or die(mysql_error($db));
if (mysql_num_rows($result)==1) {
echo fuck;
$_SESSION['username']=$user_name;
$_SESSION['logged']=1;
echo fuck;
header ('Refresh: 5;Location:index.php');
}
else{
$_SESSION['username']='';
$_SESSION['logged']=0;
echo You entered in a wrong user name or password, please check. If you want to register, click here.;
}
}
?>
我在开头就设置了session_start();
在这之前没有任何输出，但是提示我HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.如果session_start正确的话，fuck应该是可以打出来的。
求指教。
另：这是我自己写的登录模块，不知道有没有安全方面的隐患？
谢谢

------解决方案--------------------
打开 php 的错误显示功能，你会看到这一行出致命错
echo You entered in a wrong user name or password, please check. If you want to register, click here.;


我的异常网推荐解决方案：The server encountered an internal error () that prevented it from fulfilling this request.,http://www.myexception.cn/java-web/317.html