captcha (alpha numeric) in php

<?php
$RandomStr = md5(microtime());// md5 to generate the random string
$ResultStr = substr($RandomStr,0,8);
$im = @imagecreate(110, 20)
or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 0, 0, 0);
$text_color = imagecolorallocate($im, 233, 14, 91);
imagestring($im, 1, 5, 5, $RandomStr, $text_color);
imagejpeg($im,$ResultStr.'.jpg');
imagedestroy($im);
//echo '’.$randval.”;
?>