How to Use Captcha my code

[Zippo]

How can I Use Captcha in my SendComment code ??my Captcha Code :\[code\]if( empty($_SESSION['6_letters_code'] ) || strcasecmp($_SESSION['6_letters_code'], $_POST['6_letters_code']) != 0) { echo $msg .= '<script type="text/javascript">alert("Please Enter Letters Correctly .")</script>'; }\[/code\]SendComment code: \[code\]function sendcomment(){var cname = document.getElementById('sugname').value;var qplace = document.getElementById('amountqplace').innerHTML;var qfood = document.getElementById('amountqfood').innerHTML;var vfood = document.getElementById('amountvfood').innerHTML;var text = document.getElementById('sugtext').value;var datacom = "cname=" + cname +"&qplace=" + qplace + "&qfood=" + qfood + "&vfood=" + vfood + "&text=" + text;loadXMLDoc('comment.php?id=<?php echo($uid);?>&type=1','',datacom,1,"Comment Sent Successfully.");\[/code\]I Want To enter captcha in my SendComment code , if The captcha code enter wrong or didn't enter, Alert "Please Enter Letters Correctly " , and if entered correctly Alert " comment Sent Successfully. "