PHP Header & http

This article discusses how to completely eliminate the Warning:cannot add header information-headers already sent in ... This is a puzzling mistake.

As long as you have written PHP code, I believe you have encountered this most of the time is not warning it. We're going to take care of it today .....

Read the PHP manual, the answer is as follows:

Message "Warning:cannot send session Cookie-headers already sent ... "or" Cannot add header information-headers already sent ... ”。

function header (), Setcookie (), and the session function need to add header information to the output stream. But header information can only be sent before any other output. You cannot have any output (such as HTML) before using these functions. The function headers_sent () is able to check if your script has sent header information. See Output control functions.

Do not use the above function before there are any text, blank lines, carriage returns, spaces and so on. But... The problem is that the answer is not satisfactory. Because often the program in other PHP environment is normal operation.


First of all: How did this error arise? Let's take a look at how PHP handles HTTP header output and body output.

When the PHP script starts executing, it can send header (header) and principal information at the same time. Header information (from the 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 headings (such as content-type headings). However, once the script sends any non-caption output (for example, using HTML or print () calls), PHP must first send all headers and then terminate the HTTP header. And then continue sending the principal data. From this point on, any attempt to add or modify header information is disallowed and sends one of the above error messages.

Well, then we'll fix it:

Stupid method: Do not show the error warning all!
The concrete method does not say the plan ^_^

Solution:

1 is suitable for editing PHP with permission. INI's People

Open PHP. INI file (you try to test your PHP better than I do). Where is the INI), find

output_buffering = change to On or any number. If it is IIS6, be sure to ON, otherwise your PHP efficiency will be very slow.

2 Use a virtual host, you cannot edit PHP. INI, how to do?

Simple:

Create one in the root directory of your space. htaccess file, which reads as follows:

AllowOverride All
Php_flag output_buffering on

The unfortunate situation is: still not? All pages are not displayed?

Well, you can call and scold a space trader, and then let him give you Apache's. htaccess allowoverride Open

3 in the PHP file to solve

Ob_start ()
Enable the output buffering mechanism. Output Buffering supports multiple tiers-for example, the Ob_start () function can be called multiple times.

Ob_end_flush ()
Sends output buffer (output buffering) and disables the output buffering mechanism.

Ob_end_clean ()
Clears output buffer but does not send, and disables output buffering.

Ob_get_contents ()
Returns the current output buffer to a string. Allows you to process any output that the script emits.

Principle:

When output_buffering is enabled, PHP does not send HTTP headers when the script sends output. Instead, it enters the output through the pipeline (pipe) into the dynamically incremented cache (only PHP 4). 0, it has a centralized output mechanism). You can still modify/add headers, or set cookies, because the header is not actually sent. When all scripts are terminated, PHP automatically sends the HTTP header to the browser and then sends the contents of the output buffer.

Two
How the session Works


Now let's take a look at how the session works. I wonder if you know how to authenticate by cookie. First, a unique cookie is generated as the identity of the user and registered in the database. The cookie that is passed by the user is then compared with the cookie registered in the database to determine the user's identity