How to create CAPTCHA in PHP

<?php
//Start the session so we can store what the code actually is.
session_start();

$font = 'frhnd575.ttf'; //replace this with own font .ttf

//Now let's use md5 to generate a totally random string
/*
$md5 = strtoupper(md5(microtime() * mktime()));
We dont need a 32 character long string so we trim it down to 5
$string1 = substr($md5,0,8);
*/


$lenght=6;
$char_to_use='123456789';
for($i=0; $i < $lenght; $i++){
$do=mt_rand(0, strlen($char_to_use)-1);
$string=$string . $char_to_use{$do};
}
/*
Now for the GD stuff, for ease of use lets create
 the image from a background image.
*/

$captcha = imagecreatefromjpeg("captcha.jpg"); create an image named "captcha.jpg" in the same directory

/*
Lets set the colours, the colour $line is used to generate lines.
 Using a blue misty colours. The colour codes are in RGB
*/

$black = imagecolorallocate($captcha, 0, 0, 255);
$line = imagecolorallocate($captcha,0,0,0);

/*
Now to make it a little bit harder for any bots to break,
assuming they can break it so far. Lets add some lines
in (static lines) to attempt to make the bots life a little harder
*/
imageline($captcha,0,0,39,43,$line);
imageline($captcha,40,0,64,43,$line);
imageline($captcha,70,0,100,43,$line);
imageline($captcha,100,6,0,43,$line);
/*
Now for the all important writing of the randomly generated string to the image.
*/
imagettftext($captcha, 30, 0, 8, 30, $black, $font, $string);

/*
Encrypt and store the key inside of a session
*/

$_SESSION['key'] = md5($string);

/*
Output the image
*/
header("Content-type: image/png");
imagepng($captcha);
?>

Save the above code as captcha.php and call it like
<img src="captcha.php" /> <input type="text" name="cap" />

Then, when form is submitted
<?
$cap = md5($_POST['cap']);
if(!strcasecmp($cap, $_SESSION['key']) == 0):
echo "<font color=\"#FF0000\">* Sorry, your image verification was wrong!</font>";
?>

Your comments will be highly welcomed.

PHP Function To Generate DropDown FormField

<?
function generateFormField($evt,$data,$name,$width,$emp,$type,$val){
$d = split(',',$data); //split data separated by comma
$c = count($d); //count how many data passed as array after split
sort($d); // sort data like AA, AB
$drop = ''; //initialize the dropdown variable

if($evt):
$evt = " onchange=\"return $evt;\"";
endif;
$drop .= "\n<select name=\"$name\" style=\"width:$width\"$evt>\n"; //dropdown start tag
if($emp==1):
$drop .= "<option value=\"\"></option>\n"; //this empty option will be displayed if $emp=1 from the function properties
endif;

//you don't need to edit the below lines
if($type == 1):
for($i=0; $i<$c; $i++){
if($d[$i]==$_REQUEST[$name] || $d[$i]==$val):
$drop .= "<option value=\"$d[$i]\" selected=\"selected\">$d[$i]</option>\n";
else:
$drop .= "<option value=\"$d[$i]\">$d[$i]</option>\n";
endif;
}

elseif($type == 2):
for($i=0; $i<$c; $i++){
$v = split('=>',$d[$i]);
if($v[1]==$_REQUEST[$name]):
$drop .= "<option value=\"$v[1]\" selected=\"selected\">$v[0]</option>\n";
else:
$drop .= "<option value=\"$v[1]\">$v[0]</option>\n";
endif;
}
endif;
$drop .= "</select>\n";
echo $drop;
}
?>

How to use the function?
<?
/*
$evt = event, incase your dropdown has an event call onChange *optional
$data = is the main values passed to the function to populate the dropdown **required
$name = the name of the dropdown to access its value by your program **required
$width = the width of the box in pixel *optional
$emp = empty, if the value of $emp=1 from the function you will get an empty option in your dropdown, else put 0 **required
$type = data array type (Indexed Array=1 and Associate Array=2) **required
$val = you can postback the selected value to the function. *optional
*/

//examples
generateFormField('eventOnChange(this.value)',"John,Jim",'demo','130px',1,1,$_POST['demo']);//Indexed array
generateFormField('',"Silver=>150,Gold=>300",'package','130px',0,2);//Associate array
?>