Your xpath query is wrong, it should be:
$q="//div[starts-with(@id,empReview_)]/h2/h3/meta[1]/@content";
(You are missing 'h2'). Your code should look like this...
$html = new DOMDocument();
$html->loadHtml('
<div id="empReview_2055942" class="employerReview" itemscope="" itemtype="http://schema.org/Review">
<h2 class="summary">
<h3 class="review-microdata-heading">
<span class="gdRatingStars"> </span>
Former
<span itemprop="author">IT Engineer Intern in Santa Clarita, CA</span>
<meta content="4" itemprop="reviewRating"/>
</h3> <!-- presumably your input has closing tags -->
</h2>
</div>
<div id="empReview_2055947" class="employerReview" itemscope="" itemtype="http://schema.org/Review">
<h2 class="summary">
<h3 class="review-microdata-heading">
<span class="gdRatingStars"> </span>
Former
<span itemprop="author">Some Other Random Thing</span>
<meta content="7" itemprop="reviewRating"/>
</h3>
</h2>
</div>
');
$xpath = new DOMXPath( $html );
//Query to pull all reviews on the page
$q="//div[starts-with(@id,empReview_)]/h2/h3/meta[1]/@content";
$nodelist = $xpath->query($q);
foreach ($nodelist as $n){
echo $n->nodeValue;
echo"<br><br>\n";
}
I get the following output:
4<br><br>
7<br><br>