Adding variable number of zeroes to make a six digit number

[Cafspourafory]

I am working with a membership database which records all accounts in a six digit format however, some users will be using a sub-six digit format due to older conventions. So I want to be able to accept a sub-six digit number and prefix zeroes to the beginning of it.e.g.user enters number 1234, I want PHP to format it to become 001234.user enters number 123, I want PHP to format it to become 000123.