You can use file permissions to isolate things on a single virtual machine (EC2 instance). You can have separate PHP pools, and they can run as different users according to this DO tutorial. That tutorial is pretty much a guide to exactly what you need to do, and is probably going to be better than using ECS.
You should be looking at t2 instances, t2.micro might be a little small with multiple pools, but a small or medium could work for you. You'll of course have to do some testing. You can add virtual memory to EC2 instance, but there are downsides, especially since the disk is across a network on t2 instances. Other instances types have local disks, the previous generation M3/C3 instances, but you're probably best off with t2 or current generation instances with enough RAM.
Alternately look at the AWS EC2 Container Service, aka managed docker. That will let you keep things completely separate, at the expense of running multiple copies of resource intensive apps like PHP, which uses a ton of memory. A micro instance probably wouldn't cut it for many containers, but a t2.large or similar is likely suitable.
Update - follow up questions
Nginx is a fast, efficient web server and reverse proxy, that uses very little memory and is relatively easy to configure. Apache is larger and does more with the many modules available, but uses more resources.
I host 5 websites and MySQL on a t2.nano, but I occasionally run out of RAM doing a big yum update. A t2.micro is probably fine for you initially, but it depends how many PHP pools you have - they can be quite memory intensive. I have a guide on how to configure MySQL to use minimal RAM. Start with the t2.micro and the RDS service, both on the free tier, that way your database runs on a separate machine, saving RAM. Don't be surprised if you have to go up to a larger instance size due to RAM requirements.
Update from comment
@tero Kilkanen points out the following very valuable point
If you want more separation between PHP workers, you can consider
using the chroot feature of PHP-FPM. With chroot, each PHP process can
see only access the directories in its chroot. However, it needs much
more work to set up and I don't know if there are any guides to do