bezpecnostny kod

I have used following function to generate Security Images to authenticate users are real humans. Need to have GD graphic library with FreeType enabled.

Used a gif file as background, put random text on it & creates a new gif file as output.
<?php
function createImage(){
//        creates the images, writes the file
       $fileRand = md5(rand(100000,999999));
       $string_a = array("A","B","C","D","E","F","G","H","J","K",
"L","M","N","P","R","S","T","U","V","W","X","Y","Z",
"2","3","4","5","6","7","8","9");
       $keys = array_rand($string_a, 6);
       foreach($keys as $n=>$v){
           $string .= $string_a[$v];
         }
$backgroundimage = "security_background.gif";
$im=imagecreatefromgif($backgroundimage);
$colour = imagecolorallocate($im, rand(0,255), rand(0,255), rand(0,255));
$font = 'Arial';
$angle = rand(-5,5);
// Add the text
imagettftext($im, 16, $angle, 15, 25, $colour, $font, $string);
$outfile= "$fileRand.gif";
imagegif($im,$outfile);
return $outfile;
}
echo "<IMG SRC=".createImage()." name=secimg>";

?>

<form action="uloz.php" method="post" name="gbook">
    <textarea name="text"></textarea>
    <label for="ukrysa" id="ukrylabel">Odpovedz, koľko je dva krát dva &rarr;</label>
    <input name="ukrysa" id="ukrysa" type="text" maxlength="1">
    <br>
    <input type="submit" value="Pridávam sa do debaty">
</form>
<script language="JavaScript" type="text/javascript"><!--
document.gbook.ukrysa.value = '4';
document.getElementById("ukrylabel").style.display = 'none';
document.gbook.ukrysa.style.display = 'none';
//--></script>