Problem calling method inside same method in php?

Will any one please tell me how to run this class. I am getting the FATAL ERROR: Fatal error: Call to undefined function readnumber() in E:\Program Files\xampp\htdocs\numberToWords\numberToWords.php on line 20 while giving input as 120

<?php 
    class Test
    {
        function readnumber($num, $depth)
        {
            $num = (int)$num;
            $retval ="";

            if ($num < 0) // if it's any other negative, just flip it and call again

                return "negative " + readnumber(-$num, 0);
            if ($num > 99) // 100 and above
            {
                if ($num > 999) // 1000 and higher
                    $retval .= readnumber($num/1000, $depth+3);
                $num %= 1000; // now we just need the last three digits
                if ($num > 99) // as long as the first digit is not zero
                    $retval .= readnumber($num/100, 2)." hundred\n";
                $retval .=readnumber($num%100, 1); // our last two digits                       
            }
            else // from 0 to 99
            {
                $mod = floor($num / 10);
                if ($mod == 0) // ones place
                {
                    if ($num == 1) $retval.="one";
                    else if ($num == 2) $retval.="two";
                    else if ($num == 3) $retval.="three";
            开发者_运维技巧        else if ($num == 4) $retval.="four";
                    else if ($num == 5) $retval.="five";
                    else if ($num == 6) $retval.="six";
                    else if ($num == 7) $retval.="seven";
                    else if ($num == 8) $retval.="eight";
                    else if ($num == 9) $retval.="nine";
                }
                else if ($mod == 1) // if there's a one in the ten's place
                {
                    if ($num == 10) $retval.="ten";
                    else if ($num == 11) $retval.="eleven";
                    else if ($num == 12) $retval.="twelve";
                    else if ($num == 13) $retval.="thirteen";
                    else if ($num == 14) $retval.="fourteen";
                    else if ($num == 15) $retval.="fifteen";
                    else if ($num == 16) $retval.="sixteen";
                    else if ($num == 17) $retval.="seventeen";
                    else if ($num == 18) $retval.="eighteen";
                    else if ($num == 19) $retval.="nineteen";
                }
                else // if there's a different number in the ten's place
                {
                    if ($mod == 2) $retval.="twenty ";
                    else if ($mod == 3) $retval.="thirty ";
                    else if ($mod == 4) $retval.="forty ";
                    else if ($mod == 5) $retval.="fifty ";
                    else if ($mod == 6) $retval.="sixty ";
                    else if ($mod == 7) $retval.="seventy ";
                    else if ($mod == 8) $retval.="eighty ";
                    else if ($mod == 9) $retval.="ninety ";
                    if (($num % 10) != 0)
                    {
                        $retval = rtrim($retval); //get rid of space at end
                        $retval .= "-";
                    }
                    $retval.=readnumber($num % 10, 0);
                }
            }

            if ($num != 0)
            {
                if ($depth == 3)
                    $retval.=" thousand\n";
                else if ($depth == 6)
                    $retval.=" million\n";
                if ($depth == 9)
                    $retval.=" billion\n";
            }
            return $retval;
        }

    }

    $objTest = new Test();

    $objTest->readnumber(120,0);
    ?>

You are using the readnumber function in the class itself, try this instead where it appears $this->readnumber