Where can I change max_allowed_packet?

Hello

When I check /etc/my.cnf - and search for the "max_allowed_packet" line I do not find it.

I get these errors and think if I change the  "max_allowed_packet"  would be solved.

 PHP Warning: Error while sending QUERY packet. PID=16515 in ****** on line 1924, referer:
End of script output before headers: index.php,

How can I fix that?

Thanks

I have "End of script output before headers" error too.

I increased max_allowed_packet to 500 M and memory_limit = 256M but no result.