<?php

$result = <<<DATA
<body>
<a class="secondary-header-action" href="/subscribers" role="menuitem">
<span class="nav-text">Some text here</span>
</a>
</body>
DATA;

$dom = new DOMDocument('1.0', 'UTF-8');
$dom->loadHTML($result, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

$xpath = new DOMXPath($dom);
$atags = $xpath->query('//a[@class="secondary-header-action"]/span[@class="nav-text"]');
$res = array();

foreach($atags as $a) { 
   array_push($res, $a->nodeValue);
}

print_r($res);