How can i get data from one field to php function

I want to get an amount input value from field ID ‘34’ for converting to my local text in field ID ‘37’. eg. In excel can use bahttext function. Now I already have a php function. How can I get a value from field ID ‘34’ to php function and display in field ID ‘37’ before submission?

If you want it displayed live on the page, in field 37 after it is entered in field, 34, you would not use PHP (that is server side) but would you JavaScript or jQuery (that is client side.) Something like this:

There is also a plugin which can do this for you without any code (it uses jQuery):

Thank you very much for letting me know!