wordpress get image form post

functions.php

function get_post_image() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
    $first_img = $matches [1] [0];

    if(empty($first_img)){
        $img_dir = get_bloginfo('template_directory');
        $first_img = $img_dir . '/images/pf1.jpg';
    }
    return $first_img;
}