I need some help with php math functions

I have added a php order form to my website at the following


I have added a total deposit to pay now field

Two issues, one is I want the total to be divided into 2 and then that figure displayed in the total deposit to pay now input field

Second issue is after filling the details and clicking submit, I have made extra text saying total deposit to pay now, I want that to display the total price divided by 2 and then the paypal button to know that number divided into 2 and taken through to PayPal

At the moment the paypal button is just taking the total price and going through to PayPal

I can send over php files if that is possible through PM or by email if anyone wants to email me at ianhaney@irhwebsites.co.uk

Thank you in advance

Kind regards