function base64decode( data ) {
    // Decodes string using MIME base64 algorithm  
    // 
    // version: 903.3016
    // discuss at: http://phpjs.org/functions/base64_decode
    // +   original by: Tyler Akins (http://rumkin.com)
    // +   improved by: Thunder.m
    // +      input by: Aman Gupta
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   bugfixed by: Onno Marsman
    // +   bugfixed by: Pellentesque Malesuada
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +      input by: Brett Zamir (http://brettz9.blogspot.com)
    // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // -    depends on: utf8_decode
    // *     example 1: base64_decode('S2V2aW4gdmFuIFpvbm5ldmVsZA==');
    // *     returns 1: 'Kevin van Zonneveld'
    // mozilla has this native
    // - but breaks in 2.0.0.12!
    //if (typeof window['btoa'] == 'function') {
    //    return btoa(data);
    //}

    var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    var o1, o2, o3, h1, h2, h3, h4, bits, i = 0, ac = 0, dec = "", tmp_arr = [];

    if (!data) {
        return data;
    }

    data += '';

    do {  // unpack four hexets into three octets using index points in b64
        h1 = b64.indexOf(data.charAt(i++));
        h2 = b64.indexOf(data.charAt(i++));
        h3 = b64.indexOf(data.charAt(i++));
        h4 = b64.indexOf(data.charAt(i++));

        bits = h1<<18 | h2<<12 | h3<<6 | h4;

        o1 = bits>>16 & 0xff;
        o2 = bits>>8 & 0xff;
        o3 = bits & 0xff;

        if (h3 == 64) {
            tmp_arr[ac++] = String.fromCharCode(o1);
        } else if (h4 == 64) {
            tmp_arr[ac++] = String.fromCharCode(o1, o2);
        } else {
            tmp_arr[ac++] = String.fromCharCode(o1, o2, o3);
        }
    } while (i < data.length);

    dec = tmp_arr.join('');
    //dec = utf8_decode(dec);

    return dec;
}

<input type="checkbox" name="not_robot" value="1" tabindex="4" />

<script type="text/javascript">document.write(base64decode('<?php echo base64_encode('<input type="checkbox" name="not_robot" value="1" tabindex="4" />'); ?>'));</script>

<script type="text/javascript">
function base64_decode(data) {	
	var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
	var o1, o2, o3, h1, h2, h3, h4, bits, i=0, enc='';

	do {  // unpack four hexets into three octets using index points in b64
		h1 = b64.indexOf(data.charAt(i++));
		h2 = b64.indexOf(data.charAt(i++));
		h3 = b64.indexOf(data.charAt(i++));
		h4 = b64.indexOf(data.charAt(i++));

		bits = h1<<18 | h2<<12 | h3<<6 | h4;

		o1 = bits>>16 & 0xff;
		o2 = bits>>8 & 0xff;
		o3 = bits & 0xff;

		if (h3 == 64)	  enc += String.fromCharCode(o1);
		else if (h4 == 64) enc += String.fromCharCode(o1, o2);
		else			   enc += String.fromCharCode(o1, o2, o3);
	} while (i < data.length);

	return enc;
}	
</script>

<script type="text/javascript">
    document.write(base64_decode('<?=base64_encode('<img src="image.php?r=<?=rand(0,100)?>" id="code-image"/>'); ?>'));
</script>

if(!isset($_SERVER['HTTP_USER_AGENT']) || !preg_match('#(msie)|(opera)|(webkit)|(mozilla)#i', $_SERVER['HTTP_USER_AGENT']))
   die();

<script type="text/javascript"
     src="http://www.google.com/recaptcha/api/challenge?k=your_public_key">
  </script>
  <noscript>
     <iframe src="http://www.google.com/recaptcha/api/noscript?k=your_public_key"
         height="300" width="500" frameborder="0"></iframe><br>
     <textarea name="recaptcha_challenge_field" rows="3" cols="40">
     </textarea>
     <input type="hidden" name="recaptcha_response_field"
         value="manual_challenge">
  </noscript>

<script type="text/javascript">
 var RecaptchaOptions = {
    theme : 'white'
 };
</script>

<?php
  require_once('recaptchalib.php');
  $privatekey = "your_private_key";
  $resp = recaptcha_check_answer ($privatekey,
                                $_SERVER["REMOTE_ADDR"],
                                $_POST["recaptcha_challenge_field"],
                                $_POST["recaptcha_response_field"]);

  if (!$resp->is_valid) {
    // What happens when the CAPTCHA was entered incorrectly
    die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." .
         "(reCAPTCHA said: " . $resp->error . ")");
  } else {
    // Your code here to handle a successful verification
  }
  ?>