HTTP is a transfer protocol used by the World Wide Web (WWW) to retrieve information from distributed servers. The HTTP model is extremely simple; the client establishes a connection to the remote server, then issues a request. The server then processes the request, returns a response, and closes the connection. Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80, but other ports can be used like 21 (FTP), 443 (HTTPS).

Requests:  The request format for HTTP is quite simple. The first line specifies an object, together with the name of an object to apply the method to. The most commonly used method is “GET”, which ask the server to send a copy of the object to the client. The client can also send a series of optional headers. The most common headers are “Accept”, which tells the server which object types the client can handle, and “User-Agent”, which gives the implementation name of the client.

Responses: The response format is also quite simple. Responses start with a status line indicating which version of HTTP the server is running, together with a result code and an optional message. This is followed by a series of optional object headers; the most important of these are “Content-Type”, which describes the type of the object being returned, and “Content-Length”, which indicates the length.The headers are teminated by an empty line.  The server now sends any requested data. After the data have been sent, the server drops the connection.

Headers: Headers are short fragments of text which are generated by servers to hold information pertaining to each transfer as it occurs. There are four kinds of headers:

1. General-Header: These are few header fields which have general applicability for both request and response messages.  These header fields apply only to the message being transmitted which include Cache-Control, Connection, Date, Pragma, Transfer-Encoding etc.
2. Request-Header: The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server.  These fields acts as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.  The request-headers are:  Accept-Charset, Authorization, Accept-Encoding, From, Host, If-Match.
3. Response-Header: The response-header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line.  These header fields give information about the server and about futher access to the resource identified by the Request-URI. The response-header are: Age, ETag, Location, Retry-After, Server, Vary, WWW-Authenticate etc.
4. Entity-Header: Request and Response message MAY transfer an entity if not otherwise restricted by the request method or response status code.  Entity-Header fields define meta information about the entity-body or, if no body is present, about the resource identified by the request. Some of this meta information is OPTIONAL, some might be REQUIRED.  The entity-header includes: Content-Encoding, Content-Language, Content-Type, Expires, Last-Modified etc.

Status Code and Reason Phrase

The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason- Phrase.

The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:

• 1xx: Informational - Request received, continuing process.
• 2xx: Success - The action was successfully received, understood, and accepted
• 3xx: Redirection - Further action must be taken in order to complete the request
• 4xx: Client Error - The request contains bad syntax or cannot be fulfilled
• 5xx: Server Error - The server failed to fulfill an apparently valid request

 To check the headers visit: HTTP / HTTPS Header Check

Font

Use:

font: 1em/1.5em bold italic serif
…instead of
font-size: 1em; line-height: 1.5em; font-weight: bold; font-style: italic; font-family: serif

Border

Use:

border: 1px black solid
…instead of
border-width: 1px; border-color: black; border-style: solid

Background

Use:

background: #fff url(image.gif) no-repeat top left
…instead of
background-color: #fff; background-image: url(image.gif); background-repeat: no-repeat; background-position: top left;

Margin, padding, border

Use:

margin: 2px 1px 3px 4px (top, right, bottom, left)
…instead of
margin-top: 2px; margin-right: 1px; margin-bottom: 3px; margin-right: 4px

Use:

margin: 5em 1em 3em (top, left and right, bottom)
…instead of
margin-top: 5em; margin-bottom: 1em; margin-right: 1em; margin-right: 4em

Use:

margin: 5% 1% (top and bottom, left and right)
…instead of
margin-top: 5%; margin-bottom: 5%; margin-right: 1%; margin-right: 1%

These rules can be applied to margin, border and padding.

1. If a method can be static, declare it static. Speed improvement is by a factor of 4.
2. echo is faster than print and print_r.
3. Use echo’s multiple parameters instead of string concatenation.
4. Set the maxvalue for your for-loops before and not in the loop.
5. Unset your variables to free memory, especially large arrays.
6. Avoid magic like __get, __set, __autoload.
7. require_once()/include_once are expensive., use require/include instead.
8. Use full paths in includes and requires, less time spent on resolving the OS paths. Always use full path file path when opening files, to avoid expensive normalization of the file’s path. This is also important even if you use opcode cache, since the paths of includes and normal files will still need to be resolved.
9. If you need to find out the time when the script started executing, INSERT:CONTENT:END SERVER['REQUEST_TIME'] is preferred to time()
10. See if you can use strncasecmp, strpbrk and stripos instead of regex
11. str_replace is faster than preg_replace, but strtr is faster than str_replace by a factor of 4
12. If the function, such as string replacement function, accepts both arrays and single characters as arguments, and if your argument list is not too long, consider writing a few redundant replacement statements, passing one character at a time, instead of one line of code that accepts arrays as search and replace arguments.
13. It’s better to use select statements than multi if, else if, statements.
14. Error suppression with @ is very slow.
15. Turn on apache’s mod_deflate.
16. Close your database connections when you’re done with them.
17. $row['id'] is 7 times faster than $row[id]
18. Error messages are expensive.  Do use error_reporting (E_ALL) during debug.
19. Do not use functions inside of for loop, such as for ($x=0; $x < count($array); $x) The count() function gets called each time.
20. Incrementing a local variable in a method is the fastest. Nearly the same as calling a local variable in a function.
21. Incrementing a global variable is 2 times slow than a local var.
22. Incrementing an object property (eg. $this->prop++) is 3 times slower than a local variable.
23. Incrementing an undefined local variable is 9-10 times slower than a pre-initialized one.
24. Just declaring a global variable without using it in a function also slows things down (by about the same amount as incrementing a local var). PHP probably does a check to see if the global exists.
25. Method invocation appears to be independent of the number of methods defined in the class because I added 10 more methods to the test class (before and after the test method) with no change in performance.
26. Methods in derived classes run faster than ones defined in the base class.
27. A function call with one parameter and an empty function body takes about the same time as doing 7-8 $localvar++ operations. A similar method call is of course about 15 $localvar++ operations.
28. Surrounding your string by ‘ instead of ” will make things interpret a little faster since php looks for variables inside “…” but not inside ‘…’. Of course you can only do this when you don’t need to have variables in the string.
29. When echoing strings it’s faster to separate them by comma instead of dot. Note: This only works with echo, which is a function that can take several strings as arguments.
30. A PHP script will be served at least 2-10 times slower than a static HTML page by Apache. Try to use more static HTML pages and fewer scripts.
31. Your PHP scripts are recompiled every time unless the scripts are cached. Install a PHP caching product to typically increase performance by 25-100% by removing compile times.
    You can try these products:
    - ionCube PHP Accelerator: This has the most recognizable “brand”, and some hosting providers even offer it, but it’s offered only in binary form and the license seems internationally ambiguous (not free).
    -Turck MMCache for PHP: Has a good performance, and is GPL’d, but development appears to have ended way back in 2003.
    -afterBURNDER * Cache: A tool to speed up web applications.  This project is also dead now.
    - Zend Platform Performance Suite: A commercial suite that enchanced multi-layered caching, monitoring of PHP, Java, Apache & HTTP etc.
    - APC: Alternative PHP Cache: APC is a free, open, and robust framework for caching and optimizing PHP intermediate code.
32. Cache as much as possible. Use memcached - memcached is a high-performance memory object caching system intended to speed up dynamic web applications by alleviating database load. OP code caches are useful so that your script does not have to be compiled on every request
33. When working with strings and you need to check that the string is either of a certain length you’d understandably would want to use the strlen() function. This function is pretty quick since it’s operation does not perform any calculation but merely return the already known length of a string available in the zval structure (internal C struct used to store variables in PHP). However because strlen() is a function it is still somewhat slow because the function call requires several operations such as lowercase & hashtable lookup followed by the execution of said function. In some instance you can improve the speed of your code by using an isset() trick.
    Example:

if (strlen($foo) < 5) { echo "Foo is too short"; }
vs.
if (!isset($foo{5})) { echo "Foo is too short"; }

Calling isset() happens to be faster then strlen() because unlike strlen(), isset() is a language construct and not a function meaning that it’s execution does not require function lookups and lowercase. This means you have virtually no overhead on top of the actual code that determines the string’s length.

34. When incrementing or decrementing the value of the variable $i++ happens to be a tad slower then ++$i. This is something PHP specific and does not apply to other languages, so don’t go modifying your C or Java code thinking it’ll suddenly become faster, it won’t. ++$i happens to be faster in PHP because instead of 4 opcodes used for $i++ you only need 3. Post incrementation actually causes in the creation of a temporary var that is then incremented. While pre-incrementation increases the original value directly. This is one of the optimization that opcode optimized like Zend’s PHP optimizer. It is a still a good idea to keep in mind since not all opcode optimizers perform this optimization and there are plenty of ISPs and servers running without an opcode optimizer.
35. Not everything has to be OOP, often it is too much overhead, each method and object call consumes a lot of memory.
36. Do not implement every data structure as a class, arrays are useful, too
37. Don’t split methods too much, think, which code you will really re-use
38. You can always split the code of a method later, when needed
39. Make use of the countless predefined functions
40. If you have very time consuming functions in your code, consider writing them as C extensions
41. Profile your code. A profiler shows you, which parts of your code consumes how many time. The Xdebug debugger already contains a profiler. Profiling shows you the bottlenecks in overview
42. mod_gzip which is available as an Apache module compresses your data on the fly and can reduce the data to transfer up to 80%
43. Excellent Article about optimizing php by John Lim

In the above in would like to include few of my experience and few of point which i have learnt from internet. 

• PHP configuration directives make significant impact on performance of PHP.
  - Make sure that register_globals is disabled.
  - Disable magic_quotes_* directives.
  - Turn of expose_php.
  - Turn off register_argc_argv for non-cli SAPIs.
  - Unless absolutely necessary do not enable always_populate_raw_post_data.
• PHP supports zlib compression for cached pages.
• Session Optimization
  - Don’t used session.auto_start.
  - Do not enable session.use_trans_sid.
  - Whenever possible set session.cache_limiter to private_no_expire.
  - Assign each vhost it’s own sessions directory.
  - If possible avoid automatic garbage collection.