**Constructor**

I wouldn't extract the POST values inside the constructor. For the calculator class, it shouldn't matter where the values come from (it could be POST, GET, the database, anywhere really).

I would just pass the arguments as variables like this:

    public function __construct($number1, $number2, $prefix) {
        $this->number1 = $number1;
        $this->number2 = $number2;
        $this->prefix = $prefix;
    }

Your constructor also defined `$number1` etc without using it. If you want to stay with your approach, the assignments should look like this: `$this->number1 = $_POST['number1'];`


**Input Checking**

Right now, you just accept anything the user passes. For example:

    prefix=*&number2=0xf&number1=0xd
    -> 195
    prefix=-&number2=5
    -> -5
    prefix=*
    -> 0
    prefix=/
    -> error (because of the division by 0)

 If this is your intention, then that is fine, but otherwise you should check if all values are set and if they are all decimal numbers. 
At the least I would  check for division by 0.

**Misc**

 - I would call `prefix` `command` or `operation`.
 - it is customary to use CamelCase for class names, so 'calcdata' could be 'CalcData' (or something a bit more concrete like `BasicCalculator`).
 - your indentation is off. Any IDE can fix this, and it makes your code more readable.
 - you have unnecessary newlines (which makes your code a bit harder to read).