Suppose string $a holds
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Using DOMDocument and DOMXPath (a bit overkill if only a simple solution is needed):
$dom = new DOMDocument();
$dom->loadHTML($a);
$domx = new DOMXPath($dom);
$entries = $domx->evaluate("//p");
$arr = array();
foreach ($entries as $entry) {
$arr[] = '<' . $entry->tagName . '>' . $entry->nodeValue . '' . $entry->tagName . '>';
}
print_r($arr);