image captcha in php - php

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> </label><input type='submit' name='register' value='register' /></p>
</form>

Answer 1

On the first page
random number between 1 and 7 is generated and stored in session
form is displayed
picture in the captcha directory is displayed based on the random number
On the second page
array with acceptable answers is generated - the keys are numbers 1 and 7 and the values are arrays of acceptable answers
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>";
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();