Assuming you only want that part of $c to be centered (and there aren't any operations being preformed on that variable prior to output), you could do something like so:
$c .= '<span style="text-align: center;">';
$c .=X1plugin_title(laddermod_leaderboard.$event['title']);
$c .= '</span>';
However, I generally would find it bad practice to do it in that fashion. A much better approach would be to separate the variables (instead of concatenating everything into a single string) and handling the output later. An example of this would be:
<?
$plugin_title = X1plugin_title(laddermod_leaderboard.$event['title']);
?>
//Later in the page when handling the output
<span style="text-align: center;"><?= $plugin_title; ?></span>
Doing it this way gives you much better control over the individual variables and will make things easier down the line.
Granted I'm making a few assumptions about how you're handling everything so I may be way off base. 🙂
