CSC 2720 Building Web Applications

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CSC 2720Building Web Applications

• Getting and Setting HTTP Headers
• (With PHP Examples)

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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

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Introduction
HTTP/1.1 200 OK Date Mon, 23 May 2005 223834
GMT Server Apache/1.3.3.7 (Unix)
(Red-Hat/Linux) Last-Modified Wed, 08 Jan 2003
231155 GMT Etag "3f80f-1b6-3e1cb03b" Accept-Ran
ges bytes Content-Length 438 Connection
close Content-Type text/html charsetUTF-8 Body
of the contents goes here
The header section of a HTTP response

• The data in the HTTP header contains info about
  client/server and the data embedded in the HTTP
  body.

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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

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Obtaining HTTP Header Fields

• Header fields of the current request is stored in
  _SERVER.
• For a complete list of predefined names, please
  refer to
• http//www.php.net/manual/en/reserved.variables.s
  erver.php
• 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.

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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.)

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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

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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.

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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

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Redirecting the web client to another URL
exit() // Return immediately ?
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When a web client access the above file, it will
go to http//www.yahoo.com immeidately.

• header() must be called before any actual output
  is sent!

exit() ?
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Even with only one empty space sent, subsequent
calls to header() will fail.
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Send a different HTTP status code


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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.
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Tell the web client whether to cache the current
document or not
no-cache, must-revalidate") ?
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For the format of HTTP header fields, please
refer to HTTP/1.1 Header Field
Definitions http//www.w3.org/Protocols/rfc2616/r
fc2616-sec14.html The above example is extracted
from http//www.php.net/header
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Requesting the web client to download a file
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-Dis
position attachment" . "
filename\"imagename\"\n")
header("Content-Length fs\n")
readfile("imagename") ?
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References

• Wiki List of HTTP headers
• http//en.wikipedia.org/wiki/List_of_HTTP_headers
• HTTP/1.1 Header Field Definitions
• http//www.w3.org/Protocols/rfc2616/rfc2616-sec14.
  html
• PHP Manual
• http//www.php.net/manual/en/index.php