Multiple people are using the solution. 

More than five people are working with Zend PHP Engine in our organization.

We have not encountered issues with scalability as long as we effectively use caching to optimize performance of our web applications as well as a proper use of database connectivity.

I rate the solution's scalability as ten. We can scale it as per our requirements. We have 3000 solution users as our customers and three to four developers managing it.

I would rate the scalability of this solution a five out of ten. 

In my experience, PHP is indeed highly scalable. In one of our applications, we had to process a huge amount of traffic for video-based content which required almost a hundred AWS instances. We had configured load balancers and as soon as the traffic spiked, the application smoothly generated more instances on-demand and handled the traffic with no problems. 

We also find that PHP scales well even where there are multiple user types with different roles, all using the same app simultaneously. In this case, one user role was "artists", who were producing video content and uploading it. Then another role of "administrators", who had their own separate panel for admin and maintenance tasks. And, of course, there were the regular users who were using the application to watch videos. All in all, PHP managed fine even with all this going on with a high number of users. 

No issues with scalability encountered.