CSC 2720 Building Web Applications Getting and Setting HTTP Headers (With PHP Examples)

Outline  What kinds of data are embedded in the HTTP request/response headers?  How useful could these data be?  What can we achieve by setting HTTP response header?  PHP APIs for getting headers from HTTP request  PHP APIs for setting HTTP response headers  Examples

Introduction  The data in the HTTP header contains info about client/server and the data embedded in the HTTP body. HTTP/ OK Date: Mon, 23 May :38:34 GMT Server: Apache/ (Unix) (Red-Hat/Linux) Last-Modified: Wed, 08 Jan :11:55 GMT Etag: "3f80f-1b6-3e1cb03b" Accept-Ranges: bytes Content-Length: 438 Connection: close Content-Type: text/html; charset=UTF-8 Body of the contents goes here … The header section of a HTTP response

HTTP Request Headers  You can find out more about your client.  For examples  accept, accept-encoding, accept-language, accept-charset : Content types, compression schemes, languages, and character sets that the web client can handle.  user-agent : Info about the web client and operating system  referer : The URL of the webpage that "brings" the client to the requested page  cookie : Cookies

Obtaining HTTP Header Fields  Header fields of the current request is stored in $_SERVER.  For a complete list of predefined names, please refer to  Some of the useful info include  $_SERVER['HTTP_USER_AGENT']  Contains info about the web client  $_SERVER['HTTP_REFERER']  URL of the page (if any) which referred the web client to the current page. (May not be reliable)  Cookies should be access through $_COOKIE instead.

HTTP Response Headers  You can modify the HTTP response header to  Redirect the web client to another URL  Send a different HTTP status code  Tell the web client whether to cache the current document or not  Tell web client what language is used in the current document  Change the content type of the current document  You can use PHP to dynamically create text file, CSV file, image, etc.  Requesting the web client to download another file.  Set cookies (but in PHP, cookies should be set through $_COOKIE instead.)

Examples of HTTP 1.1 Response Headers  Cache-Control  Tells all caching mechanisms from server to client whether they may cache this object.  To tells a client not to cache the requested resource, set the its value to no-cache.  Content-Language  The language the content is in  Content-Type  The MIME type of the content being returned

Examples of HTTP 1.1 Response Headers  Expires  The time at which document should be considered as out- of-date  Last-Modified  The time in which the requested resource was last modified.  Location  To redirect the web client to a new URL  Set-Cookie  The cookies that browser should remember.

Functions for Dealing with Header Fields in the HTTP Response  header()  Set a raw HTTP header  headers_list()  Return a list of headers to be sent to the web client  The list is a numerically indexed array  headers_sent()  Return FALSE if no HTTP headers have already been sent or TRUE otherwise

<?php header('Location: exit(); // Return immediately ?> Redirecting the web client to another URL  header() must be called before any actual output is sent! When a web client access the above file, it will go to immeidately. <?php header('Location: exit(); ?> Even with only one empty space sent, subsequent calls to header() will fail.

<?php header("HTTP/ Not Found"); ?> Send a different HTTP status code Creating a custom-made page to display the error message when a requested resource cannot be found by the server. You also need to configure the web server to use your custom-made page.

<?php // HTTP/1.1 header("Cache-Control: no-cache, must-revalidate"); ?> Tell the web client whether to cache the current document or not For the format of HTTP header fields, please refer to HTTP/1.1: Header Field Definitions The above example is extracted from:

<?php $imagename = 'test.jpg'; $info = getimagesize($imagename); $fs = filesize($imagename); // Setting the mime-type of the file header("Content-type: {$info['mime']}\n"); // Send as attachment (request client to download) header("Content-Disposition: attachment;". " filename=\"$imagename\"\n"); header("Content-Length: $fs\n"); readfile("$imagename"); ?> Requesting the web client to download a file

References  Wiki: List of HTTP headers   HTTP/1.1: Header Field Definitions   PHP Manual 