Can you (or any fellow monk) please clarify the need for caching the prime numbers in this particular scenario?
The is_prime() check for the current $num does not depend on previous values (1..$num-1). I can understand the benefits of caching in other cases like fibonacci series where F(n) = F(n-1) + F(n-2). The OP is interating through N (e.g. 5000) points ($iter = 1..N) and checking is_prime($iter) on each value. The $iter values are unique and is_prime() is called once for each unique value. The data is plotted immediately and the is_prime'ness is not used further in the code.
While it is beneficial to cache the computed primes, in this particular scenario (OP's post), it is not providing any advantages. On the contrary, it ends up increasing memory usage of the application