Get category id in post page in wordpress

<?php $category = get_the_category(); echo $category[0]->cat_ID; ?>

<a href="?cat=<?php $category = get_the_category(); echo $category[0]->cat_ID; ?>">        <?php
    $category = get_the_category();
    //print_r($category);
$parent = get_cat_name($category[0]->category_parent);
if (!empty($parent)) {
echo '&raquo; ' . $parent;
} else {
echo $category[0]->cat_name;
}
?>
</a>