hp function setcookie() reports error: Warning: Cannot modify header_PHP tutorial

Today, when I used the php function setcookie() to set the value, I got the prompt: Warning: Cannot modify header information headers already sent by. I went to gg immediately and asked about it. I found N solutions. Let me sort it out. I hope it will be helpful to everyone. Helps.

When I was about to get off work, I saw a friend in the PHP discussion group talking about setting cookies. I asked him for the code and found out the reason! Report an error
Warning: Cannot modify header information – headers already sent by (output started at cookie1.php:4) in cookie1.php on line 5

ob_start();
setcookie("username","Song Yanbin",time()+3600);
echo "the username is:".$HTTP_COOKIE_VARS["username"]." ";
echo "the username is:".$_COOKIE["username"]." ";
print_r($_COOKIE);
?>
Warning: Cannot modify header information - headers already sent byCause of error
I added it at the head of the php program,
header("cache-control:no-cache,must-revalidate");
After that, the above error appeared on the page, and even after reading N information, there was no result. Today I accidentally discovered that there was something wrong with the configuration in my php.ini. I found the php.ini file under C:windows
output_buffering defaults to off. Now I set it to 4096 and it’s OK .
Used to solve the problem of display prompt error and the error message that (date + number of exported files) cannot be used as the file name.
The setcookie function must be sent before any data is sent to the browser
Based on the above restrictions, the etcookie() function often displays "Undefined index", "Cannot modify header information - headers already sent by"...etc.???, solution?Q" The method of Cannot modify header information - headers already sent by "?????" is to delay the output of the data to the server before generating the cookie. Therefore, you can add ob_start( at the front of the program. );???Function?lazy?br /> The ob_start() function is used to open the buffer. For example, if there is output before the header() function, including carriage returns, spaces, and line feeds, there will be "Header had all ready send by" Error, you can first use ob_start() to open the buffer. The data block of the PHP code and the output of echo() will enter the buffer and will not be output immediately. Of course, opening the buffer has many functions, just use your imagination. It can be summarized as follows Four o'clock:

1. Used before header()

ob_start(); //Open buffer
echo "Hellon"; //Output
header("location:index.php"); //Redirect the browser to index.php
ob_end_flush();//Output all content to the browser
?>

2. The phpinfo() function can obtain client and server information, but to save client information, the buffer method is the best choice.
ob_start(); //Open buffer
phpinfo(); //Use phpinfo function
$info=ob_get_contents(); //Get the contents of the buffer and assign it to $info
$file=fopen('info.txt','w'); //Open the file info.txt
fwrite($file,$info); //Write information to info.txt
fclose($file); //Close the file info.txt
?>

3. Static page technology
ob_start();//Open buffer
?>
All output of php page
$content = ob_get_contents();//Get all the contents output by the php page
$fp = fopen("output00001.html", "w"); //Create a file and open it for writing
fwrite($fp, $content); //Write all the contents of the php page to output00001.html, and then...
fclose($fp);
?>

4. Output code
Function run_code($code) {
If($code) {
ob_start();
eval($code);
$contents = ob_get_contents();
ob_end_clean();
}else {
echo "Error! No output";
exit();
}
return $contents;
}

Read the PHP manual and searched for the reason. Come to a conclusion

Method 1:

There are some restrictions on the use of cookies in PHP.
1. Use setcookie must be before the tag
2. Before using setcookie, you cannot use echo to input content
3. The cookie will not appear until the web page is loaded
4. setcookie must be placed before any data is output to the browser before sending
Due to the above limitations, when using the setcookie() function, you may encounter problems such as "Undefined index", "Cannot modify header information – headers already sent by"... etc. The solution is to generate a cookie before outputting the content, which can be done in the program Add the function ob_start();

at the top

ob_start: Open the output buffer
Function format: void ob_start(void)
Note: When the buffer is activated, all non-file header information from the PHP program will not be sent, but will be saved in the internal buffer. In order to output the contents of the buffer, you can use ob_end_flush() or flush() to output the contents of the buffer.

Method 2:

Solution to Warning: Cannot modify header information – headers already sent by… Some people say that writing ob_start(); at the beginning of the file fails.

Later open php.ini and set output_buffering to on. Restart appache, OK. It seems this is the solution.
Special note: (I solved the problem after reading this)

If you use utf-8 encoding, you must remove the BOM in UTF-8. This is because the utf-8 encoded file contains BOM, and php4 and 5 do not support BOM. To remove the BOM, you can open it with Notepad++ and convert it. (I solved the problem after reading this)
Use PHP's ob_start(); to control your browser cache.

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