[PHP] Time estimating for PM's/Clients

[Mark Steudel]

I am interested in finding out what standards/formulas other PHP
programmers/departments out there are using to accurately provide time
estimates on modules to their PM's/Clients. 
 
Any suggested reading books or sites?
Do time estimate theories in other langauges apply to web and php
programming or are they out dated?
 
THanks in advance.