Description:
------------
I can't use php with certain conditions "stream" and control video playing.

I have 2 php's index.php and stream.php

index.php init the session var id and store the video name crypted with md5 function

video.php recover the session and compare with the v parameter.

If I use $_SESSION and header/exit to check the data, send some bad headers and the video don't load.

If I use only header/exit or $_SESSION at once, I can play the video.

Reproduce code:
---------------
------------------------------------------------------------------
index.php
------------------------------------------------------------------

<?php
session_start();

$_SESSION['id'] = md5('video');

session_write_close();
?>

<object classid="clsid:22D6F312-B0F6-11D0-94AB-0080C74C7E95" width="406" height="406">
	<param name="Filename" value="stream.php?v=video">
	<param name="AutoStart" value="true">
	<param name="ShowControls" value="true">
	<param name="BufferingTime" value="5">
	<param name="ShowStatusBar" value="true">
	<param name="AutoSize" value="true">
	<param name="InvokeURLs" value="false">

	<embed src="stream.php?v=video"
		type="application/x-mplayer2"
		autostart="1"
		enabled="1"
		showstatusbar="1"
		showdisplay="1"
		showcontrols="1"
		pluginspage="http://www.microsoft.com/Windows/MediaPlayer/"
		width="406"
		height="406">
	</embed>
</object>

------------------------------------------------------------------
stream.php
------------------------------------------------------------------

<?php
session_start();

if (($_SESSION['id'] != md5($_GET['v'])) || !is_file($_GET['v'].'.wmv')) {
	header('Location: /');
	exit;
}

header('Content-Type: video/x-ms-wmv');
header('Content-Length: '.filesize($_GET['v'].'.wmv'));

readfile($_GET['v'].'.wmv');
?>

Expected result:
----------------
If all checks are fine the scritp must play the video.

Actual result:
--------------
If I use ($_SESSION['id'] == md5($_GET['v'])) and header/exit result, will never play correctly but all conditions are correct!! (The session id value == md5() and the file exists, never login to the if condition.

for example, in this example, the 1 number it's printed, but without the echo 1 the videos isn't player:

<?php
session_start();

if (($_SESSION['id'] != md5($_GET['v'])) || !is_file($_GET['v'].'.wmv')) {
	header('Location: /');
	exit;
}

echo 1;

header('Content-Type: video/x-ms-wmv');
header('Content-Length: '.filesize($_GET['v'].'.wmv'));

readfile($_GET['v'].'.wmv');
?>

With this example it works:

<?php
session_start();

if (!is_file($_GET['v'].'.wmv')) {
	header('Location: /');
	exit;
}

header('Content-Type: video/x-ms-wmv');
header('Content-Length: '.filesize($_GET['v'].'.wmv'));

readfile($_GET['v'].'.wmv');
?>

And with this example also works:

<?php
session_start();

if ($_SESSION['id'] != md5($_GET['v'])) {
	//header('Location: /');
	//exit;
}

header('Content-Type: video/x-ms-wmv');
header('Content-Length: '.filesize($_GET['v'].'.wmv'));

readfile($_GET['v'].'.wmv');
?>

But this example don't print the 1 and don't play the video:

<?php
session_start();

if ($_SESSION['id'] != md5($_GET['v'])) {
	echo 1;
	exit;
}

header('Content-Type: video/x-ms-wmv');
header('Content-Length: '.filesize($_GET['v'].'.wmv'));

readfile($_GET['v'].'.wmv');
?>

I don't understand why if I use $_SESSION and header/exit/die together the script don't work (but no erros).