Code

Index.php

--------------------------------------------------

<?php

try {

	$objDb = new PDO('mysql:host=localhost;dbname=cars', 'root', 'password');
	$objDb->exec('SET CHARACTER SET utf8');

	$sql = "SELECT *
			FROM `categories`
			WHERE `master` = 0";
	$statement = $objDb->query($sql);
	$list = $statement->fetchAll(PDO::FETCH_ASSOC);

} catch(PDOException $e) {
	echo 'There was a problem';
}

?>

<div id="wrapper">

	<form action="" method="post">

		<select name="gender" id="gender" class="update">
			<option value="">Select one</option>
			<?php if (!empty($list)) { ?>
				<?php foreach($list as $row) { ?>
					<option value="<?php echo $row['id']; ?>">
						<?php echo $row['name']; ?>
					</option>
				<?php } ?>
			<?php } ?>
		</select>

		<select name="category" id="category" class="update"
			disabled="disabled">
			<option value="">Please choose brand first!</option>
		</select>



	</form>

</div>


------------------------------------------

Update.php

-----------------------------------------

<?php
if (!empty($_GET['id']) && !empty($_GET['value'])) {

	$id = $_GET['id'];
	$value = $_GET['value'];

	try {

		$objDb = new PDO('mysql:host=localhost;dbname=cars', 'root', 'password');
		$objDb->exec('SET CHARACTER SET utf8');

		$sql = "SELECT *
				FROM `categories`
				WHERE `master` = ?";
		$statement = $objDb->prepare($sql);
		$statement->execute(array($value));
		$list = $statement->fetchAll(PDO::FETCH_ASSOC);

		if (!empty($list)) {

			$out = array('<option value="">Select one</option>');

			foreach($list as $row) {
				$out[] = '<option value="'.$row['id'].'">'.$row['name'].'</option>';
			}

			echo json_encode(array('error' => false, 'list' => implode('', $out)));

		} else {
			echo json_encode(array('error' => true));
		}

	} catch(PDOException $e) {
		echo json_encode(array('error' => true));
	}

} else {
	echo json_encode(array('error' => true));
}



--------------------------------------
Core.js
-------------------------------------
var formObject = {
	run : function(obj) {
		if (obj.val() === '') {
			obj.nextAll('.update').html('<option value="">----</option>').attr('disabled', true);
		} else {
			var id = obj.attr('id');
			var v = obj.val();
			jQuery.getJSON('/mod/update.php', { id : id, value : v }, function(data) {
				if (!data.error) {
					obj.next('.update').html(data.list).removeAttr('disabled');
				} else {
					obj.nextAll('.update').html('<option value="">----</option>').attr('disabled', true);
				}
			});
		}
	}
};
$(function() {

	$('.update').live('change', function() {
		formObject.run($(this));
	});

});