[PHP] Captcha

<?php
error_reporting(E_ALL);

function createcaptcha() {
	$captcha = array();
	$letters = array("A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5","6","7","8","9");
	$string_letters = array();
	srand ((double)microtime()*1000000);
	for($i=0;$i<=8;$i++) {
		$random = rand(1, 36);
		$string_letters[$i] = $letters[$random];
	}	
	$string = $string_letters[1];
	$string .= $string_letters[2];
	$string .= $string_letters[3];
	$string .= $string_letters[4];
	$string .= $string_letters[5];
	$string .= $string_letters[6];
	$string .= $string_letters[7];
	$string .= $string_letters[8];
	unset($string_letters);
	$randcol0=rand(1,255);
	$randcol1=rand(1,255);
	$randcol2=rand(1,255);
	$randcol3=rand(1,255);
	$randcol4=rand(1,255);
	$randcol5=rand(1,255);
	$randcol6=rand(1,255);
	$randcol7=rand(1,5);
	$randcol8=rand(1,255);
	$randcol9=rand(1,255);
	$randcol10=rand(1,255);

	$fontWidth = imagefontwidth($randcol7);
	$im = @imagecreatetruecolor((8*$fontWidth)+10, 20) OR die("Es konnte leider kein Captcha erstellt werden!");

	imagecolorallocate($im, 220, 220, 220); 
	$text_color = imagecolorallocate($im, $randcol2, $randcol5, $randcol0);
	$line_color = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color2 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color3 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color4 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color5 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color6 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
	$line_color7 = imagecolorallocate($im, rand(1,155),  rand(1,155),  rand(1,155));
		
	imageline($im, 0, 0, (8*$fontWidth)+10, 20, $line_color); 
	imageline($im, 0, 20, (8*$fontWidth)+10, 0, $line_color2); 
/* 	imageline($im, 0, 10, (8*$fontWidth)+10, 0, $line_color3); 
	imageline($im, 0, 0, (8*$fontWidth)+10, 10, $line_color4); 
	imageline($im, 0, 10, (8*$fontWidth)+10, 10, $line_color5);  */
	imageline($im, 0, 15, (8*$fontWidth)+10, 2, $line_color6); 
	imageline($im, 0, 1, (8*$fontWidth)+10, 9, $line_color7); 
	imagestring($im, $randcol7, 5, 5, $string, $text_color);
	imagepng($im);
	$captcha['string'] = $string;
	return $captcha;
};
header("Content-type: image/png");
$img = createcaptcha();
echo $img['string'];

?>

Die Grafik "https://www.paid4-szene.de/test.php" kann nicht angezeigt werden, weil sie Fehler enthält.

for($i=0;$i<=8;$i++) {
        $random = rand(1, 36);
        $string_letters[$i] = $letters[$random];
    }    
    $string = $string_letters[1];
    $string .= $string_letters[2];
    $string .= $string_letters[3];
    $string .= $string_letters[4];
    $string .= $string_letters[5];
    $string .= $string_letters[6];
    $string .= $string_letters[7];
    $string .= $string_letters[8];
    unset($string_letters);

$string = '';
for($i=0;$i<=8;$i++) {
        $random = rand(1, 36);
        $string .= $letters[$random];
    }