Problem with implementing a captcha script

<section>
	<label class="label">Enter characters below:</label>
	<label class="input input-captcha">
	     <img src="assets/plugins/forms/captcha/image.php?[[!generateCaptcha]]" width="100" height="35" alt="Captcha image" />
	     <input type="text" maxlength="6" name="captcha" id="captcha">
	</label>
</section>

echo time();

<?php

// Make the page validate
ini_set('session.use_trans_sid', '0');

// Create a random string, leaving out 'o' to avoid confusion with '0'
$char = strtoupper(substr(str_shuffle('abcdefghjkmnpqrstuvwxyz'), 0, 4));

// Concatenate the random string onto the random numbers
// The font 'Anorexia' doesn't have a character for '8', so the numbers will only go up to 7
// '0' is left out to avoid confusion with 'O'
$str = rand(1, 7) . rand(1, 7) . $char;

// Begin the session
session_start();

// Set the session contents
$_SESSION['captcha_id'] = $str;

?>

<script type="text/javascript">
			$(function()
			{
				// Validation
				$("#form").validate(
				{					
					// Rules for form validation
					rules:
					{	
						[...]
						captcha:
						{
							required: true,
							remote: 'assets/plugins/forms/captcha/process.php'
						}
					},
										
					// Messages for form validation
					messages:
					{
						[...]

						captcha:
						{
							required: 'Please enter characters',
							remote: 'Correct captcha is required'
						}
					},
										
					// Ajax form submition					
					submitHandler: function(form)
					{
						$(form).ajaxSubmit(
						{
							beforeSend: function()
							{
								$('#form button[type="submit"]').attr('disabled', true);
							},
							success: function()
							{
								$("#form").addClass('submited');
							}
						});
					},
					
					// Do not change code below
					errorPlacement: function(error, element)
					{
						error.insertAfter(element.parent());
					}
				});
			});			
		</script>				

<img src="[[++assets_url]]components/captcha/captcha.php" alt="Captcha Image" />

$captcha_ok = false;

$answer = $modx->getOption('captcha_answer', $_POST, '');

if ($answer == $_SESSION['veriword']) {
  $captcha_ok = true;
}