wordpress get the current category id name and slug

PHP

$category_array = get_the_category( $post_id );
var_dump($category_array ); // this should show you an array of the category details

// get the primary category by grabbing the 1st bit of the array, not tested if the 1st part is actually the primary category. 
$category_id = $category_array[0]['term_id'];
$category_name = $category_array[0]['name'];
$category_slug = $category_array[0]['slug'];
// ^^ this actually gives the error -- Uncaught Error: Cannot use object of type WP_Term as array
// so we need to access the array using -> rather than brackets as its an object i think. 

// here is the working code:
$post_id = the_ID();
$category_array = get_the_category( $post_id );
$category_id = $category_array[0]->term_id;
$category_name = $category_array[0]->name;
$category_slug = $category_array[0]->slug;
var_dump($category_id);

echo "<pre>";
var_dump($category_array);
echo "</pre>";