Nginx – Add custom variables to nginx access log from PHP-FPM

With the log_format its possible to include a range of variables such as $http_user_agent:

http://nginx.org/en/docs/http/ngx_http_core_module.html#variables

But is it possible to include a variable from PHP-FPM?

For example, I would like to include the User ID and timing information in the access_log.

One possibility is to set a header in PHP, and then use $sent_http_XXX:

log_format inc_info '... [$sent_http_x_user_id] ...';
access_log /var/log/nginx/access.log inc_info;

<?php
  header('X-User-ID: ' . head(USER_ID));
?>

Where you should probably use proxy_hide_header to stop those headers being sent to the client.

But this header based approach doesn't work if the headers have already been sent. For example, I use register_shutdown_function() in PHP to log the processing time.

As an aside, if you did just want the processing time, then $upstream_response_time might be useful.

In comparison, this can be done in Apache via:

LogFormat "... [%{USER_ID}n] ..." inc_info
CustomLog /var/log/httpd/access_log inc_info

With the corresponding PHP:

if (function_exists('apache_note')) {
    apache_note('USER_ID', USER_ID);
}

Although apache_note() is only available in mod_php, and it does not work with PHP-FPM.