Made a little tweak

Hello.

I had a problem with runing this module on my server because the function
imagepng($im, $filename);
and IMG tag
"<img src=\"$filename\" alt=\"Captcha Image: you will need to recognize the text in it.\"/>");
use the same variable $filename
But function imagepng uses server root folder as root in the file path
Apache uses site documents root as root folder in the link.

I made some changes in this script

   /* output to browser*/
    $siterootpath = $_SERVER['DOCUMENT_ROOT'];
    $realfilename = "$siterootpath$filename";
    imagepng($im, $realfilename);
    ImageDestroy($im);
    $output .= form_item("",
                "<img src=\"$filename\" alt=\"Captcha Image: you will need to recognize the text in it.\"/>");