Nevím jestli jde o PHP. Celková kód je takto:

add_filter( 'the_content', 'featured_image_in_content_add_to_content' );
function featured_image_in_content_add_to_content( $content ) {
if ( is_singular() && has_post_thumbnail() ) {
return get_the_post_thumbnail( null, 'large') . $content;
} else {
return $content;
}
}