preg_replace_callback returning same as input

Well I tried your code and it seemed to work as expected, even though I do see that you're missing an escape in front of your first closing square brace. I thought that may throw things off, but to my surprise it worked.

Here is the data that I used:

CREATE TABLE IF NOT EXISTS `posts` (
  `id` int(9) unsigned NOT NULL AUTO_INCREMENT,
  `tid` int(9) unsigned NOT NULL DEFAULT '0',
  `post_title` varchar(100) NOT NULL,
  `post_data` text NOT NULL,
  PRIMARY KEY (`id`),
  KEY `topic_id` (`tid`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=latin1;

INSERT INTO `posts` (`id`, `tid`, `post_title`, `post_data`) VALUES
    (12, 44, 'Nan took it', 'There once was a man from Nantucket who kept all his cash in a bucket.');

I took your input string and modified your function a bit just to make sure that it was, in fact, connecting to and pulling data from the database. The code looks like this:

$pdo = new PDO("mysql:host=localhost;dbname=so;","root","");

$post = '[smartquote]44_12[/smartquote]';


$post = preg_replace_callback("/\[smartquote]((?:(?!\[smartquote\]).)+?)\[\/smartquote\]/s", function($match) use ($pdo){
    if(isset($match[0])){
        $returnData = explode('_',$match[1]);
        if(empty($returnData[0]) || empty($returnData[1])){
            return 'Nothing found';
        } else {
            $getPost = $pdo->prepare("SELECT * FROM `posts` WHERE `tid`=:topic_id && `id`=:id");
                $getPost->bindValue(':topic_id',$returnData[0]);
                $getPost->bindValue(':id',$returnData[1]);
            $getPost->execute();
            if($getPost->rowCount() == 0){
                return '<div class="well"><h5>Quote:</h5><div style="font-size: 12px;">Quote Data</div></div>';
            } else {
                        //return 'Something, something, darkside.';

                        $row = $getPost->fetchAll();

                        $post_title = $row[0]['post_title'];
                        $post_data = $row[0]['post_data'];

                        $return_data = "<b>".$post_title."</b><br><br>".$post_data;

                        return $return_data;
                }
        }
    }            
}, $post);

print $post;

This outputs the following:

<b>Nan took it</b><br><br>There once was a man from Nantucket who kept all his cash in a bucket.

Anyway, my suggestion would be to try and escape that first closing square brace and try it again. If the data is returning the same as what you put in, that likely means that it is unable to make a match in the first place. If that is in fact the case, then you may try rewriting your expression to see if you can get it to find a match another way.

Depending on the data you are expecting, this may work and would store the topic id in $match[1] and the id in $match[2]:

~(?<=\[smartquote\])(?:(\d+)_(\d+))(?=\[/smartquote\])~s

Then you should be able to omit this part altogether:

if(isset($match[0])){
    $returnData = explode('_',$match[1]);
        if(empty($returnData[0]) || empty($returnData[1])){
        return 'Nothing found';
} else {

Since it won't run the function at all if $match[0] isn't set. Maybe you'd still like to check if $match[1] and $match[2] contain data, though.