Using Cookies

Part:   1  2  3  4 

(Continued from previous part...)

I clicked the refresh button on the IE window, I got:

Cookies added by the server:
   Cookie_2: My cookie value

Cookies received by the server:
   Cookie_1 = My cookie value

What happened here was that when I opened the page the first time, the server received no cookie from the browser's request. But my page added one cookie named as "Cookie_1" to the response.

When I clicked the refresh button, the browser sent my cookie back to the server in the request. Then my page added another cookie named as "Cookie_2" in the response.

If I keep clicking the refresh button, more and more cookies would be added to the request and response. But there is a limit. The browser will only take up to 20 cookies from one Web server.

Output Control Functions

As you can see from setcookie() definition, the PHP engine provides no buffer for the HTTP response body. That means as soon the PHP script starts to send output to the HTTP response body, the HTTP header block will be finalized, and no allowed to change.

But this default behavior can be altered by calling output control functions:

• setcookie() must be called before any output to the HTTP response. The main reason is that PHP is not buffering the HTTP response. But you can alter this behavior by using ob_*() functions.
• ob_start() - A built-in function that turns on output buffering.
• flush() - A built-in function that flushes out the contents of the output buffer to the HTTP response body.

Of course, default behavior can also be altered by the configuration file, php.ini. Open php.ini and set the following line:

output_buffering = 4096

The above configuration line tells the PHP engine to turn on output buffering, and set the buffer size to 4096 bytes. Once "output_buffering" is turned on, you don't have to call ob_start() in your scripts.

To test the PHP engine default behavior, I modified CookieTest.php into CookieOutputBuffer.php:

<?php #CookieOutputBuffer.php
# Copyright (c) 2005 by Dr. Herong Yang, http://www.herongyang.com/
#
   print("<pre>\n");
   print("Adding cookies by the server:\n");

   $numCookies = count( array_keys($_COOKIE) );
   $numCookies++;
   $cookieName = "Cookie_$numCookies";
   $cookieValue = "My cookie value";
   print("   $cookieName: $cookieValue\n");

   setcookie($cookieName, $cookieValue);

   print("\nCookies received by the server:\n");
   foreach ($_COOKIE as $k => $v) {
      print "   $k = $v\n";
   }
   
   print "</pre>\n";
?>

I then opened php.ini and set the following line:

output_buffering = 0

Running IE on CookieOutputBuffer.php gave me this:

Adding cookies by the server:
   Cookie_2: My cookie value

Cookies received by the server:
   User = Herong Yang

PHP Warning: Cannot modify header information - headers already 
sent by (output started at ...\CookieOutputBuffer.php:4) ...

Now I truly beblieve that PHP engine's default behavior is no output buffering. Make sure to change "output_buffering" back to 4096 before continuing to the next test.

(Continued on next part...)

Part:   1  2  3  4