What about using multiple rand() functions to create unique ID

I was just wondering, in term of security, what about using multiple rand() functions to generate one unique ID?

Like so: $unique_id = rand(1, 15) . rand(15, 50) . rand(50, 100) . rand(15, 50); would result to something like 8215236.

I don't want to use time().

Do you think the risk of collision is high if I use multiple rand while generating unique IDs?