image.php

 <?php
// Font directory + font name
$font = 'fonts/Disney.ttf';
// Total number of lines
$lineCount = 40;
// Size of the font
$fontSize = 40;
// Height of the image
$height = 50;
// Width of the image
$width = 150;
$img_handle = imagecreate ($width, $height) or die ("Cannot Create image");
// Set the Background Color RGB
$backColor = imagecolorallocate($img_handle, 255, 255, 255);
// Set the Line Color RGB
$lineColor = imagecolorallocate($img_handle, 175, 238, 238);
// Set the Text Color RGB
$txtColor = imagecolorallocate($img_handle, 135, 206, 235);

// Do not edit below this point
$string = "abcdefghijklmnopqrstuvwxyz0123456789";
$str = "";
for($i=0;$i<6;$i++){
    $pos = rand(0,36);
    $str .= $string{$pos};
}
$textbox = imagettfbbox($fontSize, 0, $font, $str) or die('Error in imagettfbbox function');
$x = ($width - $textbox[4])/2;
$y = ($height - $textbox[5])/2;
imagettftext($img_handle, $fontSize, 0, $x, $y, $txtColor, $font , $str) or die('Error in imagettftext function');
for($i=0;$i<$lineCount;$i++){
    $x1 = rand(0,$width);$x2 = rand(0,$width);
    $y1 = rand(0,$width);$y2 = rand(0,$width);
    imageline($img_handle,$x1,$y1,$x2,$y2,$lineColor);
}
header('Content-Type: image/jpeg');
imagejpeg($img_handle,NULL,100);
imagedestroy($img_handle);

session_start();
$_SESSION['img_number'] = $str;
?> 

form.php

<form action="result.php" method="post">
    <img alt="Random Number" src="image.php">
    <input type="text" name="num"><br>
    <input type="submit" name="submit" value="Check">
</form>

result.php

 <?php
session_start();
if($_SESSION['img_number'] != $_POST['num']){
    echo'The number you entered doesn\'t match the image.<br>
    <a href="form.php">Try Again</a><br>';
}else{
    echo'The numbers Match!<br>
    <a href="form.php">Try Again</a><br>';
}
?>
[url=http://wwww.com/examples/CAPTCHA/form2.php]Example[/url]