Display Primary Category (Yoast's WordPress SEO)

primary_category.php

<?php 
/**
 * Get the Yoast primary category from its post meta value, and displays it, with HTML markup.
 * If there is no primary category set, it displays the first assigned category.
 * 
 * @param boolean $useCatLink Whether to link the category, if it exists
 * @return void
 */
function yourtheme_display_yoast_primary_category( $useCatLink = true ) {

	// Retrieves post categories
	$category = get_the_category();

	// If post has a category assigned.
	if ( $category ) {
		$category_display = '';
		$category_link = '';
		
		// Get post's 'Primary' category from post meta
		$yoast_primary_key = get_post_meta( get_the_id(), '_yoast_wpseo_primary_category', TRUE );

		if ( !empty($yoast_primary_key) )
		{
			$term = get_term( $yoast_primary_key );

			if ( is_wp_error($term) ) { 
				// Default to first category (not Yoast) if an error is returned
				$category_display = $category[0]->name;
				$category_link = get_category_link( $category[0]->term_id );
			} else { 
				// Yoast's Primary category
				$category_display = $term->name;
				$category_link = get_category_link( $term->term_id );
			}
		}
		else {
			// Default, display the first category in WP's list of assigned categories
			$category_display = $category[0]->name;
			$category_link = get_category_link( $category[0]->term_id );
		}

		// Display category
		if ( !empty($category_display) ){
			if ( $useCatLink == true && !empty($category_link) ){
				echo '<span class="post-category">';
				echo '<a href="', esc_url($category_link), '">', esc_html_e($category_display), '</a>';
				echo '</span>';
			} else {
				echo '<span class="post-category">', esc_html_e($category_display), '</span>';
			}
		}
	}
}

?>