Calculation is not working

I made a calculator but only an addition operation is working.

Here is code

            <form>
            <input type = "text" name = "num1"/>
            <input type = "text" name = "num2"/>
            <select name = "opr">
            <option>Add</option>
            <option>MULTIPLY</option>
            <option>SUBTRACT</option>
            <option>DIVIDE</option>
            </select>
            <br>
            <button type ="submit" name ="submit" value = "submit">submit</button>
            </form>
            <?php
            if(isset($_GET['submit']))
            {
            $num1 = $_GET["num1"];
            $num2 = $_GET["num2"];
            $opr =  $_GET["opr"];

            if($opr = "ADD")
            {
                $num3 = $num1+$num2;
                echo $num3;
            }
            elseif($opr = "MULTIPLY" )
            {
                $num3 = $num1*$num2;
                echo $num3;
            }
            elseif($opr = "SUBTRACT" )
            {
                $num3 = $num1-$num2;
                echo $num3;
            }
            else{
                $num3 = $num1/$num2;
                echo $num3;
            }
            }

            ?>

I also made a conditional statement for other operations too but now sure why it's not working. I am trying to get values from URL using a $_GET function. But only ADD method is working.