header location not working in my php code

i have this code,why my header location not working? its a form of updating and editing and deleting some pages in my control panel...and i have an index.php file in the same folder of form.php...any help please?()i tryed to put the header after the editing and deleting...and still go to the form page not the index...

<?php
include "../../includes/site_includes.php";
//send
if ((isset($_POST["send"])) && ($_POST["send"] == 1)) {
    $pageid = $_POST["page_id"];
    $pagetitle = $_POST["page_title"];
    $nameinmenu = $_POST["page_menu_name"];
    $nameinurl = $_POST["page_name_url"];
    $link = $_POST["page_link"];
    $picture = $_POST["page_pic"];
    $desc = $_POST["page_desc"];
    $content = $_POST["page_content"];
}
if ((isset($_POST["act"])) && ($_POST["act"] == "add")) {
    $sql = insertpage();
    if ($result = $mysqli->prepare($sql)) {
        $result->bind_param("sssssss", $pagetitle, $nameinmenu, $nameinurl, $link, $picture, $desc, $content);
        $result->execute();
        $result->store_result();
        $rows = $result->num_rows;
    }
}
////edit
if ((isset($_GET["act"])) && ($_GET["act"] == "edit")) {
    $sql = getfrompages();
    if ($result = $mysqli->prepare($sql)) {
        $rekza = $_GET["id"];
        $result->bind_param("i", $rekza);
        $result->execute();
        $result->store_result();
        $rowsZ = $result->num_rows;
    }
    if ($rowsZ > 0) {
        $row = fetch($result);
        $pageid = $row[0]["page_id"];
        $pagetitle = $row[0]["page_title"];
        $nameinmenu = $row[0]["page_menu_name"];
        $nameinurl = $row[0]["page_name_url"];
        $link = $row[0]["page_link"];
        $picture = $row[0]["page_pic"];
        $desc = $row[0]["page_desc"];
        $content = $row[0]["page_content"];
    }
}
if ((isset($_GET["act"])) && ($_GET["act"] == "delete")) {
    $thedelid = $_GET["id"];
    $sql2 = delpage();
    if ($result2 = $mysqli->prepare($sql2)) {
        $result2->bind_param("i", $thedelid);
        $result2->execute();
        $result2->store_result();
        $rowsZ2 = $result2->num_rows;
    }
}
header('location: index.php');
exit();
?>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
 <head>
  <title> pages add </title>
  <meta name="Generator" content="EditPlus">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">
 </head>

 <body>
<form method="post" action="">
        <table>
            <tr>
                <td style="font-weight:bold;">title</td>
                <td><input type="text" name="page_title" value="<?=$pagetitle?>" /></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">name in menu</td>
                <td><input type="text" name="page_menu_name" value="<?=$nameinmenu?>" /></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">name in url</td>
                <td><input type="text" name="page_name_url" value="<?=$nameinurl?>" /></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">link</td>
                <td><input type="text" name="page_link" value="<?=$link?>" /></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">picture</td>
                <td><input type="text" name="page_pic" value="<?=$picture?>" /></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">description</td>
                <td><textarea name="page_desc"><?=$desc?></textarea></td>
            </tr>
            <tr>
                <td style="font-weight:bold;">content</td>
                <td><textarea name="page_content"><?=$content?></textarea></td>
            </tr>
            <tr>
                <td colspan="2">
                <input type="hidden" name="send" value="1" />
                <input type="hidden" name="act" value="<?=$_GET["act"]?>" />
                <input type="hidden" name="page_id" value="<?=$pageid?>" />
                <input type="submit" value="add" /></td>
            </tr>
        </table>
</form>
 </body>
</html>

solved: with @ Mihai Iorga code i added ob_start();

Solution

That is because you have an output:

?>
<?php

results in blank line output.

header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP

Combine all your PHP codes and make sure you don't have any spaces at the beginning of the file.

also after header('location: index.php'); add exit(); if you have any other scripts bellow.

Also move your redirect header after the last if.

If there is content, then you can also redirect by injecting javascript:

<?php
    echo "<script>window.location.href='target.php';</script>";
    exit;
?>