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<?php
/**
 * Добавить тумбочки к RSS
 * @param $content
 * @return string
 */
function rss_post_thumbnail( $content ) {
	global $post;
	if( has_post_thumbnail( $post->ID ) ) {
		$content = '<p>' . get_the_post_thumbnail( $post->ID, 'full' ) . '</p>' . $content;
	}
	return $content;
}
add_filter( 'the_excerpt_rss', 'rss_post_thumbnail' );
add_filter( 'the_content_feed', 'rss_post_thumbnail' );
?>