Fixing PHP Warning - 'Cannot modify header information'

I run a large site with many php files and my error_log is being filled with hundreds of thousands of lines of the following:

"Cannot modify header information - headers already sent in Unknown on line 0"

Code (markup):

I know what usually causes the issue and have checked the start of all php files with a hex editor to remove any utf8 characters.

I have also checked all headers and setcookies are before any text output on all pages.

What makes it strange is the error/warning is never output on the screen yet it's appearing thousands of times an hour for all users. I'm really struggling to find this, does anyone have any ideas on what else could be causing it? Does that fact that it's unknown on line 0 help at all?


EDIT

This warning does not occur if the following line is commented out:

php_value output_handler ob_gzhandler

Code (markup):

but why would this cause the above warnings?

 

edit your php.inichangedisplay_errors = Offanddisplay_startup_errors = Offboth off

 

Will that just stop the errors being logged?

 

log_errors = On to offBut I think you should just change the error_reporting instead of change log_errors to off

 

But I want to try and locate the source of the problem instead of simply stopping the errors being displayed.

 

is that all same error in all pages "Unknown on line 0" ?

 

The error doesn't appear on any pages it's only in the log every few seconds ..

 

change your error_reporting to E_ALL and restart webserver, you should be able to see which file access and contain that error

 

It still shows exactly the same, I sometimes get a referrer but this doesn't help find what's causing the problem.

 

If I comment this line out in .htaccess the error stops?

 

Try this.... Write ob_start(); at the very first line of your php page and write ob_flush(); on the same page where your php code ends

 

Additionally, are you sure the file has not been saved as UTF-8 w/ BOM ?

 

Put at starting
ob_start();