むー？

global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+?src=[\'"]([^"]+)[\'"].*?>/i', $post->post_content, $matches);//"
$first_img = str_replace( "http://hoge.com", "",$matches[1][0]);
if(empty($first_img)){
$first_img = "/p_sample.jpg";
}
echo $first_img;

これはすばらしい。
まーべらすまーべらす

下はだめ

$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);