A really old captcha system I found somewhere in my old codes.

captcha.php

<?php 
//Simple Captcha Function
//Creates funny and not that easily readable captchas
// ================
// (c) 2009 by DarkDevine
// Modificate as you want, just keep some credits
// Fonts from http://urbanfonts.com , you can get new ones there as well
// ================
// Usage: generate_captcha( $mode ) for generating the captcha,
// check_captcha( $string) for checking with user input

define( 'CAPTCHA_COLOR_POOL', serialize( array(			//Array of color values ( array( R, G, B ) )
			array( 134, 134, 134 ),
			array( 162, 162, 162 ),
			array( 189, 189, 189 ),
			array( 120, 120, 120 ),
			array( 98, 98, 98 ),
			array( 60, 60, 60 )
			) ) );

if( !defined( 'CAPTCHA_BACKGROUND_POOL' ) )
	define( 'CAPTCHA_BACKGROUND_POOL', serialize( array(	//Array of color values ( array( R, G, B ) )
			array( 243, 243, 243 )
			) ) );
			
define( 'CAPTCHA_CHAR_POOL', 'abcdefghkmnprtwxyABCDEFGHJKLMNPRTWXY346789' );	//Just put the chars you want to allow

define( 'CAPTCHA_SESSION_KEY', '__captcha__' );		//Key of the session variable that will be set
define( 'CAPTCHA_CHECK_CASE', FALSE );	//Choose wether or not it should be checked case-sensitive (true = case-sensitive)
define( 'CAPTCHA_FONTS', serialize( array(		//Fonts we want to use (Need to be TTF Files)
			'captcha_fonts/adler.ttf',
			'captcha_fonts/Beccaria.ttf',
			'captcha_fonts/bulkeyRefuse.ttf',
			'captcha_fonts/carbon.ttf',
			'captcha_fonts/FACERG__.TTF',
			'captcha_fonts/HALDANOR.TTF'
)));

define( 'CAPTCHA_WIDTH', 200 );				//Width of the captcha
define( 'CAPTCHA_HEIGHT', 50 );				//Height of the captcha
define( 'CAPTCHA_TEXT_LENGHT', 6 );			//Lenght of the text of the captcha
define( 'CAPTCHA_UNREADABALIZER', 30 );		//Indicates how much we wanna make the captcha...unreadable

//Function.
// $mode 0 = Return as a PNG File with a PNG Header
// $mode 1 = Return as a GIF File with a GIF Header
// $mode 2 = Return as HTML that shows the captcha
// $mode 3 = Return as Path to the image file
// $mode 4 = Return the image handle
function generate_captcha( $mode = 0 ) {
	//Checking if we installed the GD
	if( !extension_loaded( 'gd' ) )
		die( 'Failed to load PHP extension gd' );
	
	//Generate the main image
	$img = imagecreatetruecolor( CAPTCHA_WIDTH, CAPTCHA_HEIGHT );
	
	//Create a random hash
	$hash = '';
	$pool = CAPTCHA_CHAR_POOL;
	for( $x = 0; $x < CAPTCHA_TEXT_LENGHT; $x++ )
		$hash .= $pool[ rand( 0, strlen( $pool ) - 1 ) ];
	
	//Generate the background
	$bgcolors = unserialize( CAPTCHA_BACKGROUND_POOL );
	$c = $bgcolors[ rand( 0, count( $bgcolors ) - 1 ) ];
	$bgcolor = imagecolorallocate( $img, $c[ 0 ], $c[ 1 ], $c[ 2 ] );
	//Background color rectangle
	imagefilledrectangle( $img, 0, 0, CAPTCHA_WIDTH, CAPTCHA_HEIGHT, $bgcolor );
	//NEED MOAR
	//creating some random lines...
	for( $x = 0; $x < CAPTCHA_UNREADABALIZER; $x++ ) {
		$c = $bgcolors[ rand( 0, count( $bgcolors ) - 1 ) ];
		$linecolor = imagecolorallocate( $img, $c[ 0 ], $c[ 1 ], $c[ 2 ] );
		//Draw it
		imageline( $img, rand( 0, CAPTCHA_WIDTH ), 0, rand( 0, CAPTCHA_WIDTH ), CAPTCHA_HEIGHT, $linecolor );
		imageline( $img, 0, rand( 0, CAPTCHA_HEIGHT ), CAPTCHA_WIDTH, rand( 0, CAPTCHA_HEIGHT ), $linecolor );
	}
	
	//Every letter will get another font.
	//Warning, I don't implement stuff to check if the font exists, I rather write this comment instead
	$fontpool = unserialize( CAPTCHA_FONTS );
	$fontcolors = unserialize( CAPTCHA_COLOR_POOL );
	$offset = CAPTCHA_WIDTH-(CAPTCHA_WIDTH*0.8);
	for( $x = 0; $x < strlen( $hash ); $x++ ) {
		//What font will we use today?
		$font = $fontpool[ rand( 0, count( $fontpool ) - 1 ) ];
		//And what color?
		$c = $fontcolors[ rand( 0, count( $fontcolors ) - 1) ];
		$color = imagecolorallocate( $img, $c[ 0 ], $c[ 1 ], $c[ 2 ] );
		//Height modifier
		$hm = rand( -((int)(CAPTCHA_UNREADABALIZER/20)), (int)(CAPTCHA_UNREADABALIZER/20) );
		//Font size
		$fs = rand( (int)(CAPTCHA_WIDTH/10), (int)((CAPTCHA_WIDTH/10)+(CAPTCHA_UNREADABALIZER/7)) );
		//Angle
		$angle = rand( -15, 15 );
		//Get the bounding box
		$b = imagettfbbox( $fs, $angle, $font, $hash[ $x ] );
		//Calculate the Y Position with the height of the font, the image height and the modifier
		$y = (CAPTCHA_HEIGHT/1.5)-((($b[ 1 ]-$b[ 6 ])/2)+$hm);
		//Finally...write the char
		imagettftext( $img, $fs, $angle, $offset, $y, $color, $font, $hash[ $x ] );
		//and move the offset forward
		$offset += $b[ 4 ] - $b[ 6 ];		
	}
	//Trash it up a bit more
	for( $x = 0; $x < (int)(CAPTCHA_UNREADABALIZER/10); $x++ ) {
		$c = $bgcolors[ rand( 0, count( $bgcolors ) - 1 ) ];
		$linecolor = imagecolorallocate( $img, $c[ 0 ], $c[ 1 ], $c[ 2 ] );
		//Draw it
		imageline( $img, rand( 0, CAPTCHA_WIDTH ), 0, rand( 0, CAPTCHA_WIDTH ), CAPTCHA_HEIGHT, $linecolor );
		imageline( $img, 0, rand( 0, CAPTCHA_HEIGHT ), CAPTCHA_WIDTH, rand( 0, CAPTCHA_HEIGHT ), $linecolor );
	}
	
	//And finally fuck the whole mess even more up
	imagefilter( $img, IMG_FILTER_GAUSSIAN_BLUR );

	//Saving our nice hash/trash to a session variable
	if( !isset( $_SESSION ) )
		@session_start();
	$_SESSION[ CAPTCHA_SESSION_KEY ] = $hash;
	
	//Do some garbage cleaning
	if( $mode == 2 || $mode == 3 ) {
		$files = glob( './_cptmp/*_*.png' );
		if( count( $files ) ) {
			foreach( $files as $file )
				if( time()-filemtime( $file ) > 60 )
					unlink( $file );
		}
	}
	
	switch( $mode ) {
		case 0:
		default:
			header( 'Content-type: image/png' );
			imagepng( $img );
			break;
		case 1:
			header( 'Content-type: image/gif' );
			imagegif( $img );
			break;
		case 2:
			//Save the damn thing somewhere
			if( !is_dir( './_cptmp' ) )
				mkdir( './_cptmp', 0777, true );
			$path = './_cptmp/'.md5( $hash ).'.png';
			imagepng( $img, $path );
			echo '<img src="'.$path.'" border="0" />';
			break;
		case 3:
			if( !is_dir( './_cptmp' ) )
				mkdir( './_cptmp', 0777, true );
			$path = './cptmp/'.md5( $hash ).'.png';
			imagepng( $img, $path );
			imagedestroy( $img );
			return $path;
		case 4:
			return $img;
	}
	imagedestroy( $img );
}

function check_captcha( $string ) {
	if( !isset( $_SESSION[ CAPTCHA_SESSION_KEY ] ) )
		return FALSE;
	return CAPTCHA_CHECK_CASE === TRUE ? ( strcmp( $string, $_SESSION[ CAPTCHA_SESSION_KEY ] ) != 0 ? FALSE : TRUE ) : ( strtolower( $string ) == strtolower( $_SESSION[ CAPTCHA_SESSION_KEY ] ) ? TRUE : FALSE );
}

?>