PHI = Permanent Health Insurance (better known as Income Continuance or Income Protection). As an employee, PHI might be deducted from your salary by your payroll department and you'll only pay PRSI/ Health Levy on the income after deduction of PHI. If it isn't deducted from your salary you need to do a tax return after the tax year ends and claim the relevant relief yourself.
I think the original poster referred to private Health Insurance eg VHI/ BUPA/ VIVAS. Somewhat confusingly, this is not PHI! These payments don't affect either PRSI or the Health Levy calculations.