Data attribute value from MySQL

Data attribute value from MySQL

I'm new to PHP and MySQL so i'm not quite sure what i am doing wrong here.
I am using the jQuery plugin quicksand to create a filterable portfolio.
The plugin uses custom data attribute data-tag inside the li item to
populate the filter nav.
What I am trying to do is use a foreach loop to populate the contents of
the ul. The problem is that the filter nav won't auto populate when I use
php to get the value of the data attribute from the gallery_tag column in
my mySQL table.
<?php
$pagetitle = "Red Penguin - Our Work";
$navcurrent = "work";
$headTitle = "RECENT WORK";
$headsubTitle = "SOME OF OUR RECENT WORK";
include_once('includes/headersub.php');
include_once('includes/connection.php');
include_once('includes/project.php');
$project = new Project;
$projects = $project->fetch_all();
?>
<div class="row">
<nav id="filter"></nav>
<section id="container">
<ul id="stage" class="three-up">
<?php foreach($projects as $project) { ?>
<li class="gallerylist" data-tag="<?php echo
$project['gallery_tag']; ?>">
<a href="project.php?id=<?php echo $project['gallery_id'];
?>">
<img src="<?php echo $project['gallery_thumb']; ?> "
alt="<?php echo $project['gallery_proj']; ?>" />
<?php echo $project['gallery_title']; ?>
</a>
</li>
<?php } ?>
</ul>
</section>
The error that comes up in the log is in the jquery line:
tags = elem.data('tags').split(',');
I'm not quite sure why this is a conflict that causes the jquery to be
unable to read the value of the data-attribute as taken from the
gallery_tag column of my table. Any help would be appreciated.