I'll update this if I find more. For now, this seems to be a partial solution. It's a Gmail add on (except I don't use Gmail) that adds reminders. The options when setting reminders include cancelling it if someone replies, and to remind everyone in the thread. If you combine those two, it seems like — if you use Gmail's web client and have $23/month you're dying to get rid of — a decent way to add the feature. Source: over 1 year ago

PHP PEAR is a repository of PHP software code, offering a vast range of packages that extend the functionality of PHP. These packages can help PHP developers streamline their projects by providing ready-to-use components. Before we delve into how to install PEAR packages, make sure your cPanel hosting plan supports PEAR and that you have the necessary permissions to make such installations. Source: 12 months ago

Importing external libraries as dependencies used to be a real pain in PHP. However, one of the most prominent changes in PHP’s maturity came with the release of Composer. Before Composer, PHP used PEAR, which solved the same problem in a more primitive way. - Source: dev.to / over 2 years ago