On 2nd thought:
dws is right, you need to use a module or something, since the approach I list below is probably not thread-safe.
You have the right idea, when you want to generate a sequence reliably, use srand($seed), keeping $seed consistent across calls to srand. Each time you call srand() this resets the random number generator. If you want to "clear" the random number generator for more random usage, just call srand() with no seed.
#!/usr/bin/pseudocode
srand($seed);
print rand, rand, rand -> 1, 2, 3
srand($seed);
print rand, rand, rand -> 1, 2, 3
srand();
print rand, rand, rand -> 42, 256, 3.14
srand($seed);
print rand, rand, rand -> 1, 2, 3
