In my servers Apache error log I regularly see the entry:

mod_fcgid: stderr: PHP Fatal error: Call to undefined function module_list() in /home/example/public_html/includes/bootstrap.inc on line 1074

Lines 1066 -> 1078 in bootstrap.inc are:

function bootstrap_invoke_all($hook) {   // Bootstrap modules should have been loaded when this function is called, so   // we don't need to tell module_list() to reset its internal list (and we   // therefore leave the first parameter at its default value of FALSE). We   // still pass in TRUE for the second parameter, though; in case this is the   // first time during the bootstrap that module_list() is called, we want to   // make sure that its internal cache is primed with the bootstrap modules   // only.   foreach (module_list(FALSE, TRUE) as $module) {     drupal_load('module', $module);     module_invoke($module, $hook);   } } 

No errors appear in Drupal’s Database logging. Today I have received some white screen 503s when checking the site and each time a new message appears in the Apache log. I have tried clearing caches and running drush cc all.

Does anyone has a suggestion as to what is causing this error or how to resolve it?

Some specifications about my set-up: PHP 5.5.12 (OPcache 7.0.4 as bundled), MariaDB 5.5.37, Apache HTTP 2.2.15, Centos 6.5 and Drupal 7.28. OPcache is ‘Up and Running’.

To check if OPcache was the causing the problem I disabled it and monitored the error log. The error message still appears with OPcache disabled, and as it disabling it slows the site site significantly (in the region of 200ms > 600 ms) I have re-enabled.