Seniors, help solve this problem

Seniors, help solve this problem
I made a login user's page, done well, in the case of a plain page, I gave it a login user authentication. The code is then generated on top of it.

if (!function_exists ("getsqlvaluestring")) {
function getsqlvaluestring ($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "")
{
if (Php_version < 6) {
$theValue = GET_MAGIC_QUOTES_GPC ()? Stripslashes ($theValue): $theValue;
}

$theValue = function_exists ("mysql_real_escape_string")? Mysql_real_escape_string ($theValue): mysql_escape_string ($theValue);

Switch ($theType) {
Case "Text":
$theValue = ($theValue! = "")? "". $theValue. "'": "NULL";
Break
Case "Long":
Case "int":
$theValue = ($theValue! = "")? Intval ($theValue): "NULL";
Break
Case "Double":
$theValue = ($theValue! = "")? Doubleval ($theValue): "NULL";
Break
Case "Date":
$theValue = ($theValue! = "")? "". $theValue. "'": "NULL";
Break
Case "defined":
$theValue = ($theValue! = "")? $theDefinedValue: $theNotDefinedValue;
Break
}
return $theValue;
}
}
?>
Validate request to login to this site.
if (!isset ($_session)) {
Session_Start ();
}

$loginFormAction = $_server[' php_self ');
if (Isset ($_get[' AccessCheck ')) {
$_session[' prevurl '] = $_get[' AccessCheck '];
}

if (isset ($_post[' username ')) {
$loginUsername =$_post[' username '];
$password =$_post[' USERPSW '];
$MM _flduserauthorization = "";
$MM _redirectloginsuccess = "admin.php";
$MM _redirectloginfailed = "index.php";
$MM _redirecttoreferrer = true;
mysql_select_db ($database _boardconn, $boardConn);

$LoginRS __query=sprintf ("Select Username, userpsw from board_admin WHERE username=%s and userpsw=%s",
Getsqlvaluestring ($loginUsername, "text"), getsqlvaluestring ($password, "text"));

$LoginRS = mysql_query ($LoginRS __query, $boardConn) or Die (Mysql_error ());
$loginFoundUser = mysql_num_rows ($LoginRS);
if ($loginFoundUser) {
$loginStrGroup = "";

if (php_version >= 5.1) {session_regenerate_id (true);} else {session_regenerate_id ();}
Declare session variables and assign them
$_session[' mm_username '] = $loginUsername;
$_session[' mm_usergroup '] = $loginStrGroup;

if (Isset ($_session[' Prevurl ')) && true) {
$MM _redirectloginsuccess = $_session[' Prevurl ');
}
Header ("Location:".) $MM _redirectloginsuccess);
}
else {
Header ("Location:".) $MM _redirectloginfailed);
}
}
?>
But I'm browsing.
This error occurs on the page:
Warning:session_start () [Function.session-start]: Cannot send session cache Limiter-headers already sent (output Starte D at E:\appserv\www\webboard2\login.php:2) in E:\AppServ\www\webBoard2\login.php on line 5

Had foreseen such a problem before, it would have Validate request to login to this site.
if (!isset ($_session)) {
Session_Start ();
}
This section of the PHP code on the line, this time do not know how still not, to help solve the next, thank you



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