Wordpress Get first image in post content if no featured image is available. (blog)

functions.php

// Get First image for Blog section


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

    if (isset($matches[1][0]))
        $first_img = $matches[1][0];

    return $first_img;
}

in-loop.php

<?php $postimg = get_first_image(); ?>
<?php

if (has_post_thumbnail()):
    the_post_thumbnail();
elseif ($postimg):
    ?>      
    <img src="<?php echo $postimg; ?>"/>        
<?php else: ?> <img src="DEFAULT IMG SRC HERE" alt=""/>

<?php endif; ?>