在PHP中的图像验证码

Question

Below is the source code of a program. Can anyone help me to figure out the working of a program.

<?php
session_start();
?>
<?php
  $aCaptcha = array (
        array(),
        array('crocodile'),
        array('panda', 'panda bear', 'giant panda'),
        array('pig'),
        array('tiger'),
        array('zebra'),
        array('cow'),
        array('elephant')
    );

if (isset($_POST['register'])) {
    $error = array();
        if (!in_array(strtolower($_POST['captcha']), $aCaptcha[$_SESSION['captcha']])) {
        $error['captcha'] = "<span style='color:red'>The name of the animal is not correct.</span>";
    }
    if (count($error) == 0) {
                    echo "<span style='color:red'>Thank you for completing the form.
We shall contact you soon.</span>";
        die();
    }
}
?>
<form action="index.php" method="post">
    <?php
            $_SESSION['captcha'] = rand(1, 7);
    ?>  
<td colspan="3"><strong>Contact Form</strong></td>

             <p>Full Name :                   <input type="text" name="Nmaes" value='' />
<p>Mobile No. :                   <input type="text" name="Nmaes" value='' />
             <p>Email id :                   <input type="text" name="Nmaes" value='' />
             <p>Subject :                   <input type="text" name="Nmaes" value='' />
 <p>Message :                   <input type="text" name="Nmaes" value='' />
    <p><img src="<?php echo $path;?>captcha/<?php echo $_SESSION['captcha'];?>.jpg" /></p>
    <p>Type the name of the animal you see in the picture above. <input type="text" name="captcha" value='' />
    <?php echo(isset($error['captcha']))?$error['captcha']:"";?></p>
    <p><label>&nbsp;</label><input type='submit' name='register' value='register' /></p>
</form>

Answer 1

On the first page

1. random number between 1 and 7 is generated and stored in session
2. form is displayed
3. picture in the captcha directory is displayed based on the random number

On the second page

1. array with acceptable answers is generated - the keys are numbers 1 and 7 and the values are arrays of acceptable answers

2. the below code checks that the answer given by the user $_POST['captcha'] is one of the acceptable answers $aCaptcha[$_SESSION['captcha']]

    if (!in_array(strtolower($_POST['captcha']), $aCaptcha[$_SESSION['captcha']])) { $error['captcha'] = "<span style='color:red'>The name of the animal is not correct.</span>"; 

3. if acceptable then a message is printed out and PHP stops executing

    echo "<span style='color:red'>Thank you for completing the form. We shall contact you soon.</span>"; die();