<?php
 
$data = <<<DATA
<li data-type="mentionable" data-id="2">bla bla... 
<li data-type="mentionable" data-id="812">some test 
<li>bla bla </li>more text 
<li data-type="mentionable" data-id="282">
DATA;
 
$doc = new DOMDocument();
$doc->loadHTML($data, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
 
$xpath = new DOMXPath($doc);
 
$ids = [];
foreach ($xpath->query("//li[@data-id]") as $item) {
	$ids[] = $item->getAttribute('data-id');
}
print_r($ids);
?>

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