WordPress get the primary category for a post (Yoast’s “Primary” Category )

primary-category.php

<?php
// Show Yoast’s “Primary” Category. Fallback to first category.
$category   = get_the_category();
$usecatlink = true;
// If post has a category assigned.
if ( $category ) {
	$category_display = '';
	$category_link    = '';
	if ( class_exists( 'WPSEO_Primary_Term' ) ) {
		// Show the post's 'Primary' category, if this Yoast feature is available, & one is set.
		$wpseo_primary_term = new WPSEO_Primary_Term( 'category', get_the_id() );
		$wpseo_primary_term = $wpseo_primary_term->get_primary_term();
		$term               = get_term( $wpseo_primary_term );
		if ( is_wp_error( $term ) ) {
			// Default to first WP category (not Yoast) if an error is returned.
			$category_display = $category[0]->name;
			$category_link    = get_category_link( $category[0]->term_id );
		} else {
			// Yoast 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 ( true === $usecatlink && ! empty( $category_link ) ) {
			echo '<span class="entry-terms category" itemprop="articleSection">';
			echo '<a href="' . esc_url( $category_link ) . '">' . esc_html( $category_display ) . '</a>';
			echo '</span>';
		} else {
			echo '<span class="entry-terms category" itemprop="articleSection">' . esc_html( $category_display ) . '</span>';
		}
	}
}