Add Advanced Custom Fields to Yoast SEO keyword density filter.

acf-yoast-seo.php

<?php

/**
 * Add ACF Fields to Keyword evaluation for Yoast SEO Plugin
 * @link https://imperativeideas.com/making-custom-fields-work-yoast-wordpress-seo/
 */

if ( is_admin() && defined('WPSEO_VERSION') ) { // check to make sure we aren't on the front end
	add_filter('wpseo_pre_analysis_post_content', 'add_acf_to_yoast');

	function add_acf_to_yoast( $content ) {
		global $post;
		$pid = $post->ID;
		$custom_content = '';

		$custom = get_post_custom($pid);
		unset($custom['_yoast_wpseo_focuskw']); // Don't count the keyword in the Yoast field!

		foreach( $custom as $key => $value ) {
			if( substr( $key, 0, 1 ) != '_' && substr( $value[0], -1) != '}' && !is_array($value[0]) && !empty($value[0])) {
			  $custom_content .= $value[0] . ' ';
			}

		}

		$content = $content . ' ' . $custom_content;
		

		return $content;

		remove_filter('wpseo_pre_analysis_post_content', 'add_acf_to_yoast'); // don't let WP execute this twice
	}
}

?>