When a PHSP is set up, the employer decides how many credits / dollars will be applied to each employee's Health Spending Account each year.  This determines the budget for health benefits at the beginning of each year.

Since a PHSP carries no premiums, you don't have to worry about premiums increasing year after year.

The "pay-as-you-claim" plan dictates that you only pay the administration fees as claims are submitted.  If no claims are submitted, you don't pay anything.