FB Post Image Fix

function insert_fb_image_fix() {
    global $post;
    if ( !is_singular())
        return;
    if(!has_post_thumbnail( $post->ID )) {

        $files = get_children('post_parent='.get_the_ID().'&post_type=attachment&post_mime_type=image&order=desc');
          if($files) :
            $keys = array_reverse(array_keys($files));
            $j=0;
            $num = $keys[$j];
            $image=wp_get_attachment_image($num, 'medium', true);
            $imagepieces = explode('"', $image);
            $imagepath = $imagepieces[1];
            $main=wp_get_attachment_url($num);
            echo '<meta property="og:image" content="' . $main . '"/>';
          endif;

    }
    else{
        $thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'large' );
        echo '<meta property="og:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
    }
    echo "";
}
add_action( 'wp_head', 'insert_fb_image_fix', 5 );