Follow this lesson in Ullman Chapter 4. The scripts are located in the 04 directory.
See the page calculator.html described on Ullman pages 68ff.
See the page handle_calc.php described on Ullman pages 71ff.
Particularly note these lines:
// In case register_globals is disabled.
$price = $_POST[‘price’];
Operator  Purpose  
$a

Negation  Opposite of $a. 
$a + $b

Addition  Sum of $a and $b. 
$a – $b

Subtraction  Difference of $a and $b. 
$a * $b

Multiplication  Product of $a and $b. 
$a / $b

Division  Quotient of $a and $b. 
$a % $b

Modulus  Remainder of $a divided by $b. 
Combined Operators

Same as:  
$a += $b

Addition  $a = $a + $b 
$a = $b

Subtraction  $a = $a – $b 
$a *= $b

Multiplication  $a = $a * $b 
$a /= $b

Division  $a = $a / $b 
$a %= $b

Modulus  $a = $a % $b 
Incrementing/Decrementing  
a++

Increment  $a = $a + 1 
a–

Decrement  $a = $a – 1 
See the page random.php.
Examine the different versions of handle_calc.php. What are the differences, and why are they significant?
Create your own page that demonstrates round() or number_format(). It should have text boxes that take a decimal number, and a number to round by.
Review Chapter 4 of Ullman.