<?php
$width = 100;
$height = 50;
$im = imagecreate($width, $height);
$bg = imagecolorallocate($im, 0, 0, 0);

// generate random string
$len = 5;
$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
$string = '';
for ($i = 0; $i < $len; $i++) {
    $pos = rand(0, strlen($chars)-1);
    $string .= $chars{$pos};
}

// grid
$grid_color = imagecolorallocate($im, 175, 0, 0);
$number_to_loop = ceil($width / 20);
for($i = 0; $i < $number_to_loop; $i++) {
    $x = ($i + 1) * 20;
    imageline($im, $x, 0, $x, $height, $grid_color);
}
$number_to_loop = ceil($height / 10);
for($i = 0; $i < $number_to_loop; $i++) {
    $y = ($i + 1) * 10;
    imageline($im, 0, $y, $width, $y, $grid_color);
}

// random lines
$line_color = imagecolorallocate($im, 130, 0, 0);
for($i = 0; $i < 30; $i++) {
    $rand_x_1 = rand(0, $width - 1);
    $rand_x_2 = rand(0, $width - 1);
    $rand_y_1 = rand(0, $height - 1);
    $rand_y_2 = rand(0, $height - 1);
    imageline($im, $rand_x_1, $rand_y_1, $rand_x_2, $rand_y_2, $line_color);
}

// write the text
$text_color = imagecolorallocate($im, 255, 0, 0);
$rand_x = rand(0, $width - 50);
$rand_y = rand(0, $height - 15);
imagestring($im, 10, $rand_x, $rand_y, $string, $text_color);

session_start();
$_SESSION['captcha'] = $string;

header ("Content-type: image/png");
imagepng($im);
?>