How To Calculate CGPA Using PHP

This is my coding that I try but I still cannot get the function to calculate cgpa.
<script language="Javascript" type="text/javascript">
/*Gred*/
function DisplayGred (GredPair) {
var Gred = GredPair.split('|');
var jam_kredit = document.getElementById('jam_kredit').value; (user input pay amount student pay for course)
document.getElementById('pointer').value = Gred[1]; (display course fee when item is selected)
}
function CalculateCGPA () .....

This is the interface look alike, the code subject, subject name and credit hours are retrive from the database, so how to calculate it?
</p>
<form id="form1" name="form1" method="post" action="">
<table width="552" height="82" border="0" class="tborder">
<tr bgcolor="#006699" class="text2">
<td width="17" bgcolor="#000099"><div align="center" class="style3"><strong><font size="2">Bil</font></strong></div></td> .....