Discussion about output buffering One of Outputbuffering_PHP Tutorial

####################################### ##Title Discussion about Output Buffering# #organizeDiegoLynn@林国国##Original workBy Zeev Suraski #################################### #### Discussion on Output Buffering (Output buffering) Contents HTTP Header Why use Output Buffering technology How Output Buffering works Basic usage Advanced usage makes things simpler Haha, I succeeded. Personally, I think Output buffering is relatively pure. PHP 4.0 features. Although conceptually quite simple, output buffering is extremely powerful and makes it easier for developers to develop advanced and efficient programs. This article will introduce HTTP headers, how output buffering can help you deal with HTTP headers, and introduce some advanced uses of output buffering. HTTP Header [HTTP header] For every request established using the HTTP protocol, the response generated by the web server usually consists of two parts - the header and the body. For example, if there is a small text file called example.txt in the document root directory of the web server, and the file contains the text Hello, world!, then the HTTP request response to this file will be as follows: HTTP/1.1 200 OK Date: Sat, 02 Sep 2000 21:40:08 GMT Server: Apache/1.3.11 (Unix) mod_macro/1.1.1 PHP/4.0.2-dev Last-Modified: Sat, 02 Sep 2000 21:39:49 GMT ETag: "12600b-e-39b173a5" Accept-Ranges: bytes Content-Length: 14 Connection: close Content-Type: text/plain Hello, world! The first part (that is, the larger part) of this request is the HTTP header. Although the HTTP header is not visible to the user in the browser, it contains information for the browser, such as the document content type, the protocol version used, the date the document was last modified, and so on. There are not many rules for HTTP headers. Usually, its format is as follows: Field: Value[Field: value] They must be separated from the document body with a blank line. This HTTP header's information can be added or changed from a PHP script. For example, you can use the header() function: header("Location: http://www.php.net/"); // Redirect to http://www.php.net/ You can also use the SetCookie() function: SetCookie("foo", "bar"); You may know that HTTP cookies are implemented using HTTP headers. For example, the HTTP request response for the following PHP file will look like this: HTTP/1.1 200 OK Date: Sat, 02 Sep 2000 21:43:02 GMT Server: Apache/1.3.11 (Unix) mod_macro/1.1. 1 PHP/4.0.2-dev X-Powered-By: PHP/4.0.2-dev Set-Cookie: foo=bar Connection: close Content-Type: text/html Set cookie. The browser reads the HTTP returned from the server header, knowing that a cookie called foo (a session cookie in this case) was sent, and its value is bar. Why use Output Buffering technology? The need for output buffering technology was obvious as early as PHP/FI 2.0. If you have used this version of PHP, you may still remember that you often encountered the Oops, SetCookie called after header has been sent error message, which made you scratch your head and wonder what the reason was. If you have used the latest version of PHP -- PHP 3.0 or even PHP 4.0 -- then you will know this error message: Oops, php_set_cookie called after header has been sent. Alternatively, you may encounter the Cannot add header information - headers already sent message when trying to call PHP's header() function. In general, output buffering technology users avoid these annoying error messages, while developers can also use it for advanced purposes. When did these errors occur? These error messages can occur if you try to add or modify header information after the HTTP header has been sent, and if there is a lack of blank lines between the document body and the header. To understand how this happens, let's take a look at how PHP handles HTTP header output and body output. When the script starts executing, it can send header information and body information at the same time. Header information (from header() or SetCookie() function) is not sent immediately, instead, it is saved to a list. This allows you to modify the header information, including the default header (such as the Content-Type header). However, once the script sends any non-header output (for example, using an HTML block or a print() call), then PHP must first send all the headers, then send a blank line, terminate the HTTP header, and then only The main data will continue to be sent. From this point on, any attempt to add or modify header information is not allowed and will send one of the above error messages. While this doesn't cause much of a problem, sometimes it just complicates the script logic by terminating the HTTP header before sending any input. Output buffering technology can solve these problems. How Output Buffering works When output buffering is enabled, PHP does not send HTTP headers when the script sends output. Instead, it pipes this output into a dynamically growing cache (only available in PHP 4.0, which has a centralized output mechanism). You can still modify, add header rows, or set cookies, since the headers are not actually sent.In the simplest case, when the script terminates, PHP will automatically send the HTTP header to the browser and then send the contents of the output buffer. It's simple. Basic usage You can use the following four functions, which can help you control output buffering: ob_start() enables the output buffering mechanism. Output buffering supports multiple levels - for example, the ob_start() function can be called multiple times. ob_end_flush() sends the output buffer (output buffer) and disables the output buffering mechanism. ob_end_clean() clears the output buffer without sending it, and disables output buffering. ob_get_contents() returns the current output buffer as a string. Allows you to process any output emitted by the script. Additionally, the output_buffering directive in php.ini can be enabled. If this directive is enabled, each PHP script is equivalent to calling the ob_start() function at the beginning. Example 1

Example 1

Here, even though you have sent the output (in the HTML code block and in the print statement), you can use the SetCookie() call without error, really Thanks to the output buffering mechanism. Please note that using the output buffering mechanism for this purpose will incur a certain performance penalty, so it is best not to enable this mechanism by default. However, for more complex scripts, output buffering can simplify the logic. Example 2 This example shows an inefficient way of determining the length of a string. Instead of simply using the strlen() function, it first enables the output buffering mechanism, prints out the string, and then determines the length of the output buffer. Finally clear the output buffer (not send), and then disable the output buffering mechanism.

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