Get Primary Category for Post Wordpress, Yoast

index.php

<?php
$primary_cat_id = get_post_meta(get_the_ID(), '_yoast_wpseo_primary_category', true);

if ( $primary_cat_id != null ) {
	$category = get_category($primary_cat_id);
} else {
	$categories = get_the_category();
	$category = $categories[0];
}