session_start() causing lots of warnings after response sent to browser

Question

EDIT

This is what the code is for:

1. The user clicks a button which starts a long-running process on the server (not actually uploading a file, that's just to get the code working)
2. If the process finishes before sending the response nothing seems to be happening on the user's browser and they might click again/reload the page/do something else undesirable
3. An immediate response is required to let the user know they have to wait
4. A progress indicator shows how far the process has got, i.e. roughly how long the user will have to wait

I just need some way of telling the user in real-time roughly what the progress on the server is. After they click, everything happens on the server until the process in finished, then they get a redirection signal and go to the next page.

If I need to throw away all my code and try something else I'll do it!

END EDIT

The question has been asked many times but I can't find an answer that actually works yet.

I'm setting up a progress indicator for the first time and I have it mostly working except for a lot of warnings. Here is the code:

<?php
session_name('test');
$filesize = 10000000;

// Allow script to send response but keep running afterwards
ignore_user_abort(true);
ob_start();
header('Connection: close');
header('Content-Length: ' . ob_get_length());
ob_end_flush();
//ob_flush();
flush();

for ($i = 0; $i < $filesize; $i += rand(4096, 8192)) { // Just testing...
  ob_start();
  @session_start();
  $_SESSION['progress'] = $i;
  session_write_close();
  ob_end_clean();
}
ob_start();
@session_start();
$_SESSION['progress'] = $filesize;
session_write_close();
ob_end_clean();
exit;
?>

I have a second script which then reads the values out of $_SESSION which is workig fine. If I remove the @s before session_start(); in the above script I get multitudes of the following error:

[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP Warning:  session_start(): Cannot send session cookie - headers already sent in C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php on line 36, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP Stack trace:, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP   1. {main}() C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php:0, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP   2. session_start() C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php:36, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP Warning:  session_start(): Cannot send session cache limiter - headers already sent in C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php on line 36, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP Stack trace:, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP   1. {main}() C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php:0, referer: http://localhost/SRI/progress.php
[Fri Aug 22 08:59:59.109375 2014] [:error] [pid 4100:tid 1248] [client 127.0.0.1:2577] PHP   2. session_start() C:\\server\\Apache24\\htdocs\\localhost\\SRI\\progress.php:36, referer: http://localhost/SRI/progress.php

I really don't like using @ as it only suppresses the warnings; I'd much rather fix them. Is there a cleaner way of getting this working?

I have already eliminated the following:

• UTF-8 BOM
• Output between flush() and the next session_start()
• Using output buffering (ob) for every session_start()
• Using pcntl_fork() (doesn't work under Apache)

The second lot of ob_start()s have helped with getting a timely response to the browser but my error log is still filling up! I suspect that since the flush() has sent headers and I'm still in the same script that this is why the warnings keep happening. I have a WAMP server for development but will be transferring the working program to a LAMP server.

Answer 1

Once you send any output in PHP, you cannot call session_start() or header(). This is because you are creating an HTTP response with your script which will end up looking something like this:

HTTP/1.1 200 OK
Server: nginx/1.6.1
Cookie: ...

// Content goes here

Once you start outputting content, you cannot output headers as that would create malformed response. Every time you call ob_end_flush() you are sending the contents of the output buffer. This means that you can't do a session_start() after it.

In your code above, you should be able to remove everything and just do:

<?php

  session_name('test');
  session_start();

  $filesize = 10000000;

  // whatever else you want to do

  $_SESSION['progress'] = $filesize;

Also make sure that there is no whitespace before the opening php bracket. This is often the culprit in getting errors when calling session_start(). Sometimes it can also happen via trailing whitespace on an included file.

Answer 2

OK, so I've fixed the problem by splitting my two scripts (process & progress) into three scripts (prepare, process & progress) called by three different parts of the same web page:

1. Send "prepare" request. Server initialises some variables and saves them in the $_SESSION. On success go to step 2
2. Send "process" request. Server starts running the long process using the variables previously stored in the $_SESSION during step 1. On success load content/redirect to next page
3. Send "progress" requests at regular intervals while step 2 is running. On success update progress indicator on the page

Since I'm using XHR/AJAX anyway this solution seems to do the trick! I no longer need the "send output but keep running" solution.