Your problem is that this function:
function CalculatePrice()
{
$var_price = $_POST['field_width'] * $_POST['field_height'];
}
in its present state is useless. $var_price is only created inside the scope of the function. Once that function has finished executing and returns to the place it was called, it's as if the variable never existed (since in that scope, it actually never has).
If you've never heard of variable scope before, see the following manual page for an introduction to the topic: [man]variables.scope[/man].
EDIT: Just in case it isn't immediately clear, the solution of using global variables is a very bad practice to get into - instead, your function above should probably be [man]return[/man]'ing the price (which you can capture in a variable for use in the scope where you call the function).
