<?php
$html = <<<HTML
<body>
<div class='post-body entry-content' id='post-body-37'>
<div style="text-align: left;">
<div style="text-align: center;">
Hi
</div></div></div>
</body>
HTML;
$dom = new DOMDocument;
$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED|LIBXML_HTML_NODEFDTD);
$xp = new DOMXPath($dom);
$links = $xp->query('//div[starts-with(@class,"post-") and contains(@class, "content")]');
foreach ($links as $link) {
	echo $link->nodeValue;    
}