open-graph.php

<?php 

function add_opengraph_doctype($output) {
    return $output . '
    xmlns="https://www.w3.org/1999/xhtml"
    xmlns:og="https://ogp.me/ns#" 
    xmlns:fb="http://www.facebook.com/2008/fbml"';
}
add_filter('language_attributes', 'add_opengraph_doctype');

//Add Open Graph Meta Info from the actual article data, or customize as necessary
	function facebook_open_graph() {
	    global $post;
	    if ( !is_singular()) //if it is not a post or a page
	        return;
		if($excerpt = $post->post_excerpt) 
	        {
 			$excerpt = strip_tags($post->post_excerpt);
			$excerpt = str_replace("", "'", $excerpt);
        	} 
        	else 
        	{
            		$excerpt = get_bloginfo('description');
		}

	    //You'll need to find you Facebook profile Id and add it as the admin
	        echo '<meta property="fb:admins" content="XXXXXXXXX-fb-admin-id"/>';
	        echo '<meta property="og:title" content="' . get_the_title() . '"/>';
			echo '<meta property="og:description" content="' . $excerpt . '"/>';
	        echo '<meta property="og:type" content="article"/>';
	        echo '<meta property="og:url" content="' . get_permalink() . '"/>';
	    //Let's also add some Twitter related meta data
	        echo '<meta name="twitter:card" content="summary" />';
	    //This is the site Twitter @username to be used at the footer of the card
			echo '<meta name="twitter:site" content="@site_user_name" />';
		//This the Twitter @username which is the creator / author of the article
			echo '<meta name="twitter:creator" content="@username_author" />';
	        
	    // Customize the below with the name of your site
	        echo '<meta property="og:site_name" content="'. get_bloginfo( 'title' ) .'"/>';
	        if(!has_post_thumbnail( $post->ID )) { //the post does not have featured image, use a default image
	    //Create a default image on your server or an image in your media library, and insert it's URL here
	        $default_image=  get_template_directory_uri() . '/dist/assets/images/homepage-slider-hero.jpg"';
	        echo '<meta property="og:image" content="' . $default_image . '"/>';
	    }
	    else{
	        $thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'medium' );
	        echo '<meta property="og:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
	    }

	    echo "
	";
	}
add_action( 'wp_head', 'facebook_open_graph', 5 );