Create Calculated Field in PHP Form

Question

Hi,
I'm trying to create a calculated field that will work out the average of ratings given during the input of a form. This field will then submit the average value with the form at the end (which is why I need the value to calculate before the form submits - i.e., as the form is filled in).
I have found the following code (designed for HTML forms rather than PHP forms, but I'm hoping it might work). It doesn't seem to do anything - the field doesn't show any information. Help?
<script language=javascript>
<!--//
function dm(amount)
  string = "" + amount;
  dec = string.length - string.indexOf('.');
  if (string.indexOf('.') == -1)
  return string + '.00';
  if (dec == 1)
  return string + '00';
  if (dec == 2)
  return string + '0';
  if (dec > 3)
  return string.substring(0,string.length-dec+3);
  return string;
function calculate()
  Opp1 = 0;  Inv1 = 0;  Com1 = 0;  wlb1 = 0;  Com2 = 0;  Atm1 = 0;  Per1 = 0;  Inc1 = 0;  Rew1 = 0;  Env1 = 0;  Gro1 = 0;
  if (document.form1.Opportunity.value > "")
     { Opp1 = document.form1.Opportunity.value };
  document.form1.Opportunity.value = eval(Opp1); 
  if (document.form1.Investment.value > "")
     { Inv1 = document.form1.Investment.value };
  document.form1.Investment.value = eval(Inv1); 
  if (document.form1.Community.value > "")
     { Com1 = document.form1.Community.value };
  document.form1.Community.value = eval(Com1); 
  if (document.form1.WLB.value > "")
     { wlb1 = document.form1.WLB.value };
  document.form1.WLB.value = eval(wlb1); 
  if (document.form1.Communication.value > "")
     { Com2 = document.form1.Communication.value };
  document.form1.Communication.value = eval(Com2); 
  if (document.form1.Atmosphere.value > "")
     { Atm1 = document.form1.Atmosphere.value };
  document.form1.Atmosphere.value = eval(Atm1); 
  if (document.form1.Performance.value > "")
     { Per1 = document.form1.Performance.value };
  document.form1.Performance.value = eval(Per1); 
  if (document.form1.Inclusion.value > "")
     { Inc1 = document.form1.Inclusion.value };
  document.form1.Inclusion.value = eval(Inc1); 
  if (document.form1.Rewards.value > "")
     { Rew1 = document.form1.Rewards.value };
  document.form1.Rewards.value = eval(Rew1); 
  if (document.form1.Environment.value > "")
     { Env1 = document.form1.Environment.value };
  document.form1.Environment.value = eval(Env1); 
  if (document.form1.Groups.value > "")
     { Gro1 = document.form1.Groups.value };
  document.form1.Groups.value = eval(Gro1); 
  Totamt =
     eval(Opp1) +
     eval(Inv1) +
     eval(Com1) +
     eval(wlb1) +
     eval(Com2) +
     eval(Atm1) +
     eval(Per1) +
     eval(Inc1) +
     eval(Rew1) +
     eval(Env1) +
     eval(Gro1) ;
  document.form1.GrandTotal.value = dm(eval(Totamt));
//-->
</script>
Any suggestions gratefully received.

Answer 1

@uscart - Thanks for your thoughts. I think I may try and pursue the javascript option (considering how far I've gone with it) but don't quite understand your response. Does it matter if the input field isn't a text box? The fields are radio buttons (giving options 1-10), liek so:
  <input name="Investment" type="radio" class="star" value="1"/>
  <input name="Investment" type="radio" class="star" value="2"/>
  <input name="Investment" type="radio" class="star" value="3"/>
  <input name="Investment" type="radio" class="star" value="4"/>
...etc, etc, etc... for each of the input fields listed above.
Where do I put the calculate() function? After each "option", or is there a different reference? And does it need to be added to the GrandTotal field as well (the field in which the calcualtion is to be displayed)?
Thank you!