The New Relic PHP agent does not support any Apache multi-processing model (MPM) that involves threading.
The PHP language development team discourages the use of a threaded MPM with Apache, and we have chosen not to implement support for threaded MPMs in the PHP agent for similar stability reasons. This includes the worker and event MPMs.
If the default, non-threaded prefork MPM is unsuitable for your needs for performance reasons, we suggest using mod_fcgid and php-fpm to use PHP via FastCGI instead.
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Additional documentation resources include: