Single file PHP Captcha generator. You can use this to request a captcha image via the tag <img src="http://your.website/captcha.php" alt="captcha"> and a form to insert the captcha code. In your destination page you only have to include the same file (with include_once('captcha.php'); ) and it will check itself if the captcha code is valid!

captcha.php

<?php

if(!isset($_SESSION)) {
	@session_start();
}

//debug
//define('_JEXEC',1);

//cosa faccio?
if(defined('_JEXEC') || defined('_VALID_MOS')) {
	$given = isset($_POST['captcha'])?$_POST['captcha']:isset($_GET['captcha'])?$_GET['captcha']:'';
	if(isset($_SESSION['captcha']) && $_SESSION['captcha']!='' && $given !== $_SESSION['captcha']) {
		?>
		<script type="text/javascript">
			alert('Errore, Ricompila questo form!');
			history.back();
		</script>
		<?php
		die('<!-- error captcha -->');
	}
	else {
		//doing nothing!
	}
}
else {
	$font_file = "./arialbd.ttf";
	$font_size = 25;
	$length = 6;

	$captcha = "";
	$salt = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
	$len = strlen($salt);
	mt_srand(10000000*(double)microtime());
	for ($i = 0; $i < $length; $i++)
		$captcha .= $salt[mt_rand(0,$len - 1)];
	$captcha = strtoupper($captcha);
	$texture = imagecreatefrompng('texture.png');

	$source = imagecolorat($texture, rand(0, imagesx($texture)), rand(0, imagesy($texture)));
	$source2 = 0x94989B;
	$r = ($source >> 16 & 0xff) + 70;
	$g = ($source >> 8  & 0xff) + 70;
	$b = ($source & 0xff) + 50;
	//$r = 87; $g = 189; $b = 263;
	$text_color = imagecolorallocate($texture, $r, $g, $b);
	//
	$r = ($source2 >> 16 & 0xff) + 70;
	$g = ($source2 >> 8  & 0xff) + 70;
	$b = ($source2 & 0xff) + 50;
	$line_color = imagecolorallocate($texture, $r, $g, $b);
	//87 - 189 - 263
	$posx = 23; //imagesx($texture)/2 - strlen($captcha) * ($font_size/3);
	$posy = imagesy($texture)/2 + $font_size/2;

	//imagestring($texture, 6, (imagesx($texture) - strlen($row['captcha']) * 5)/ 2, 5,  $captcha, $text_color);
	for($i=0;$i<$length;$i++) {
		$x = $posx + $i*$font_size*1.3;
		$y = $posy + rand(-$posy*0.3,$posy*0.3);
		//if($i>0)	imageline($texture, $x-6, $posy*0.5, $x-6, $posy*1.3, $line_color);
		imagefttext($texture, $font_size,rand(-30,30), $x, $y, $text_color, $font_file, $captcha[$i]);
	}
	
	//registro il codice
	$_SESSION['captcha'] = $captcha;
	
	//stampo l'immagine
	header('Content-Type: image/png');
	imagepng($texture);
	imagedestroy($texture);
}

?>