WordPress helper function to retrieve an attribute (or the whole category object) of the primary category. Should be used inside the lop.

get-primary-category.php

<?php
/**
 * Get Primary Category
 *
 * Get the primary category of the post.
 * Should be used inside the loop.
 *
 * @param  string  $att		Attribute of the category that is desired (if 'all' is passed, will return whole object)
 * @param  boolean $wpseo	Whether or not to use WPSEO (if it's on/installed)
 *
 * @return string/object	Either the first category or primary if WPSEO is on/installed
 */
public static function get_primary_cat( $att = 'slug', $wpseo = true ) {
	if ( class_exists('WPSEO_Primary_Term') && $wpseo !== false ) {
		$wpseo_primary_term = new WPSEO_Primary_Term( 'category', get_the_id() );
		$wpseo_primary_term = $wpseo_primary_term->get_primary_term();
		$cat                = get_category( $wpseo_primary_term );

		// WPSEO will return WP error if there is no set primary category
		if ( is_wp_error($cat) ) {
			$post_cats   = get_the_category();
			$cat         = !empty( $post_cats[0] ) ? $post_cats[0] : '';
			$cat_display = !empty( $cat->$att ) ? $cat->$att : '';

		} else {
			$cat_display = $cat->$att;
		}
	} else {

		$post_cats   = get_the_category();
		$cat         = !empty( $post_cats[0] ) ? $post_cats[0] : '';
		$cat_display = !empty( $cat->$att ) ? $cat->$att : '';

	}

	return $att != 'all' ? $cat_display : $cat;
}