How to split two values from an array?

I have the following code, where only index 0 is string:

        $row[0] = $row[0];
        $row[1] =(float) $row[1];
        $row[2] =(float) $row[2];
        $row[3] =(float) $row[3];
        $row[4] =(float) $row[4];
        $row[5] =(float) $row[5];
        $row[6] =(float) $row[6];

I need to split one index by the other, I'm trying like this:

       $a = $row[5] / $row[6];
        $row[7] =$a;

What's happening?