I have built plenty of sites with PHP, python, C++, and perl. Hands down PHP is the best at getting data in and out of some sort of data store. Some of the sites were very busy commercial sites with effectively 100s of thousands of dynamic pages for products and their associated details, etc. These sites would be peaking out around 20,000 views per minute nearing Christmas with many of those being sales transactions. These sites typically ran on under 10 dual cpu run of the mill servers with no amazing attributes. The servers were completely underwhelmed by the traffic.
Other PHP driven backends for mobile apps were serving in the range of 10000 simultaneous users who were usually hitting the servers around twice a minute with either some data to store or a request for other details. This was handled by two servers, one with redis and the other with apache and PHP. The web server was pretty good with around 16 cores on 2 CPUs.
We were all ready to jump in and C++ the crap out of any part that needed it but the measurements were showing 20ms responses on most requests. Reducing this to 0ms would not have had any benefits.
The reality that I have discovered is that the two choke points for most REST or web servers is the datastore, and the network distance to your customers. So getting a CDN or spreading your servers out closer to your customers, combined with making sure your datastore has enough RAM to do most operations in RAM is where the speed and capacity will come from. PHP has never been a problem. While C++ would buy some unnecessary speed, the development time would cost piles. Python would be slower, although I regularly use it for offline stuff such as ML or just plain old data massaging, and node.js is similar and would have similar arguments to PHP it just isn't the way I have gone.
So while you say you wouldn't use it for something more than 100 users, I don't know what on earth you would recommend other than nodejs as a replacement. What you might be thinking about is PHP when combined with some of the nightmare frameworks that the script kiddies love. Most of the frameworks that get the most press are bloated piles of steaming dogshit that are the classic solutions that will get you to 90% done and leave you hanging at 90% for the next 2 years until your startup runs out of cash. The worst being that if you used one of really crappy popular frameworks and have any success you will find scaling to pretty much be impossible and that you are throwing absurd numbers of servers at very small workloads.
To press home my PHP point, I have done simple from memory datastore websites that were completely adequate for maybe up to 1000 views per minute on a Raspberry Pi.