<?php

/* create a test image */
$im = @imagecreate(100, 20) or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 255, 255, 255);
$text_color = imagecolorallocate($im, 0, 0, 0);
imagestring($im, 1, 5, 5,  "Hello, World!", $text_color);

/***
 * Assumptions:
 *   A monochrome image where characters are black
 *   A single character is connected
 *   Characters are disjointed by white space
 */

$width = imagesx($im);
$height = imagesy($im);

/***
 * Notes:
 *   The smaller the sample size the more accurate it will be,
 *   however, it will take longer. Larger images can use a larger
 *   sample size wihtout compromizing much accuracy.
 */
$x_sample = 1;
$y_sample = 1;

$last = 0;
for($i = 0; $i < $width; $i++) {
    $col = array();

    for($j = 0; $j < $height; $j++) {
        $col[$j] = imagecolorat($im, $i, $j);
    }

    if(($current = array_sum($col)) > 0) {
        if($last == 0) {
            $l = 0;
        }

        for($k = 0; $k < $height; $k++) {
            if(($l % $x_sample) == 0) {
                if(($k % $y_sample) == 0) {
                    $sample .= $col[$k];
                }
            }
        }

        $l++;
        $last = $current;
    } else {
        $last = 0;
    }

    if(!empty($sample) && $last == 0) {
        echo $sample . "\n";
        $sample = "";
    }
}
?>

<?php
function randomString($length){
    $pattern = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
    if(empty($length)){
        $length = "10";
    }

    for($i=0; $i<$length; $i++){
        $string .= $pattern{rand(0,61)};
    }

    return $string;
}

function captcha() {
    header("Content-type: image/png");
    $im = @imagecreate(100, 50) or die("Cannot Initialize new GD image stream");
    $background_color = imagecolorallocate($im, 0, 0, 0);
    $text_color = imagecolorallocate($im, 255, 0, 0);
    $string = randomString();
    imagestring($im, 10, 5, 5,  $string, $text_color);
    imagepng($im);
    imagedestroy($im);
}

captcha();
?>