Sum of the checkbox values are basically classified into two types.

1. As direct sum of the no. of checkboxes.
2. As sum of the checkboxes at each time select the checkbox values.

Briefly on,
1.As direct sum of the no. of checkboxes.


Here you no need to write the javascript or any function, simply assign sum of checkboxes values to a variable finally print that variable using "echo" meanwhile the source code as follows:-


$sum=$x+$y+$z; //here assign $x=<input type="checkbox" name="term1" value="500" /> 
vice versa.

echo "$sum";