Excimer is an extension for PHP 7.1+ that provides a low-overhead interrupting timer and sampling profiler.
Excimer is available from packages.debian.org, and can be installed via apt-get install php-excimer.
For other installation methods, see https://www.mediawiki.org/wiki/Excimer.
For the API, see PHPDoc stubs.
For usage examples, see https://www.mediawiki.org/wiki/Excimer#Examples.