PHP caching mechanism

I often encounter this problem when writing PHP programs;

Including using the header() function, session() function, and cookies function, there may be problems

The PHP program reports the following error:

Cannot modify header information - headers already sent by (output started at

The header infomation cannot be modified. To solve this error, you need to understand two issues.

First, http protocol

In the http protocol, the server outputs html files to the browser, The html file consists of two parts. One part is the data of our page itself, and the other part is the header information of the html page. This information contains a lot of data, such as the encoding format in which the page is displayed, the size of the html data, whether to jump, and whether to cache. Wait. Once generated, these html header file information is not allowed to be modified. This is the reason for the error reported above,

Second, PHP’s caching mechanism

has two parts during the running of the php program. Cache, one is the program cache, which I understand as the data cache of the program in the memory; the other is the cache module provided by PHP. Through this module, PHP will open up an additional place as a data cache. If the PHP module cache is turned on, then when When there is echo data in the program, the data is put into the cache instead of directly generating the HTML file. Until the PHP program is finished running, the module cached data is then used to generate HTML code, including header files. Here are two examples:

1. The cache is not enabled (this is the default)

<?php
echo "123";
header("Content-type:text/html;charset=utf-8")
echo "456";
?>

The Cannot modify header information error is exposed at this time;

2. The cache mechanism is enabled

<?php
ob_start() ; //开启页面缓存
echo "123";
header("Content-type:text/html;charset=utf-8")
echo "456";
?>

In the above two examples, the cache status is not enabled. When echo "123";, an HTML code has been generated. When reaching the third line, the header function cannot modify the header file information and reports an error; In Example 2, ob_start(); turns on the cache (ob is output_buffer). When echo "123", the data is written to the cache module, and then the header() function is run. At this time, the HTML page is not generated, and then echo "456"; is also output to the cache module. When the program is executed, the data of the cache module is Generate a complete HTML page so that no errors are reported

Second, PHP’s caching mechanism function and how to enable it

As mentioned above, the ob_start() function can enable the cache module, but this function only opens this page. Yes, if necessary, you can modify the php.ini file, find the output_buffering option, and change it to output_buffering = 4096 (On is also OK). The number represents the cache size.

There are also some functions in the PHP caching mechanism. You can try it to better understand the PHP caching mechanism.

ob_start() starts output buffering. At this time, PHP stops outputting. After that, the output is transferred to an internal In the buffer.

ob_get_contents() This function returns the contents of the internal buffer. This is equivalent to turning these outputs into strings.

ob_get_ length() returns the length of the internal buffer.

ob_end_flush() ends the output buffer, And output the contents in the buffer. After this, the output is normal output.

ob_end_clean() ends the output buffer and throws away the contents in the buffer.