differences between header(location:URL) and fopen(URL)

Hi there
$url = "xx/yy/a.thml"
-----------------
header("location : $url"); // it works properly

but

$file_handle = fopen($url, "r"); // shows that your session is expired
while (!feof($file_handle))
echo fgets($file_handle, 4096);
fclose($file_handle);
----------------------
why?
thank you all

 

thats because header() function redirects user (visitor) to given location. fopen() function is run in server not in user browser so you get error.