May I ask - what are you planning on using the RNG for? Because if it's for applications like generating passwords - it might not be that useful. If there are already quantumcomputers powerful enough to predict the movement of E.Coli..they will surely enough be powerful enough to just bruteforce the passwords.
There aren't quantum computers powerful enough to predict the movement of E.Coli.
How random is PHP math_rand(0,n) ?
Depends on what purpose. If someone knows exactly what time (ms) you generated a random number using it, then they could regenerate the same "random" number. It's not 100% secure for generating things like Bitcoin addresses, but in all likelihood, you'd probably be fine using it. A true RNG has results that are unreproducible.
Predicting movement of E. coli sounds unfeasible based on general chaos theory.