Trying to prevent form re-submission upon refresh, can't modify header information

I'm trying to make the page redirect to the same page to avoid resending the same form information. However, using header('Location: guestbook.php'); gives me an error:

Warning: Cannot modify header information - headers already sent by (output started at /test/guestbook.php:1) in /test/guestbook.php on line 29

I'm not sure if I'm putting the header in the right 开发者_开发问答place, I'm not extremely familiar with using it:

<?php
$gb_str = "";   // $gb_str is the string we'll append entries to
$pgeTitle = "View and Sign Guestbook";

// If form is submitted, then insert into DB
if (!empty($HTTP_POST_VARS["submit"])) {
    // initiate some vars
    $dbHost = ;
    $dbUser = ;
    $dbPass = ;
    $dbDatabase = ;
    $li = mysql_connect($dbHost, $dbUser, $dbPass) or die("Could not connect");
    mysql_select_db($dbDatabase, $li) or die ("could not select DB"); 

    $name = mysql_real_escape_string($HTTP_POST_VARS["name"]);
    $email = mysql_real_escape_string($HTTP_POST_VARS["email"]);
    $comment = mysql_real_escape_string($HTTP_POST_VARS["comment"]);
    $date = Date("Y-m-d h:i:s");

    $gb_query =     "insert into entries
            values(0, '$name', '$email', '$comment', '$date')";

    mysql_query($gb_query);
    $res = mysql_affected_rows();

    // See if insert was successful or not
    if($res > 0) {
    $ret_str="Your guestbook entry was successfully added!";
    header('Location: guestbook.php');
    exit(); // End the request

    } else {
        $ret_str = "There was a problem with your guestbook entry. Please try again.";
    }

    // Append success/failure message
    $gb_str .= "<span class=\"ret\">$ret_str</span><BR>";
    mysql_close();
}
?>

You have BOM or just regular spaces in the beginning of the file. Just remove them

Most probably you have sent output before setting header();.
Try disabling notices (and probably warnings) and make sure you don't send any output before setting header();.

<?php
error_reporting(E_ALL ^ E_NOTICE); // Print all errors except notices - they come out for example when you're requiring an undefined var.
$gb_str = "";   // $gb_str is the string we'll append entries to
$pgeTitle = "View and Sign Guestbook";

// If form is submitted, then insert into DB
if (!empty($HTTP_POST_VARS["submit"])) {
    // initiate some vars
    $dbHost = "localhost";
    $dbUser = "someuser"; 
    $dbPass = "password";
    $dbDatabase = "database";
    $li = mysql_connect($dbHost, $dbUser, $dbPass) or die("Could not connect");
    mysql_select_db($dbDatabase, $li) or die ("could not select DB"); 

    $name = mysql_real_escape_string($HTTP_POST_VARS["name"]);
    $email = mysql_real_escape_string($HTTP_POST_VARS["email"]);
    $comment = mysql_real_escape_string($HTTP_POST_VARS["comment"]);
    $date = Date("Y-m-d h:i:s");

    $gb_query = "insert into entries values(0, '$name', '$email', '$comment', '$date')";

    mysql_query($gb_query);
    $res = mysql_affected_rows();

    // See if insert was successful or not
    if($res > 0) {
        $ret_str="Your guestbook entry was successfully added!";
        header('Location: guestbook.php');
        exit(); // End the request

    } else {
        $ret_str = "There was a problem with your guestbook entry. Please try again.";
    }

    // Append success/failure message
    $gb_str .= "<span class=\"ret\">$ret_str</span><BR>";
    mysql_close();
}
?>