pgmb_yoast_seo_variables.php

<?php

if(interface_exists('\PGMB\Placeholders\VariableInterface')){
	class PGMBYoastSEOVariables implements \PGMB\Placeholders\VariableInterface {

		private $parent_id;

		public function __construct($parent_id) {
			$this->parent_id = $parent_id;
		}

		public function variables() {
			return [
				'%yoast_wpseo_metadesc%'   => get_post_meta($this->parent_id, '_yoast_wpseo_metadesc', true),
			];
		}
	}
}

add_action('mbp_placeholder_decorators', function($decorators, $parent_post_id, $location){
	if(class_exists('PGMBYoastSEOVariables')){	
		$yoast_seo_variables = new PGMBYoastSEOVariables($parent_post_id);
		$decorators[] = $yoast_seo_variables;
	}
	
		return $decorators;
	}
,10, 3);