<?php
 
$html = <<<EOD
<form method="post" action="guardar.php">
    <input type="hidden" name="produto1" value="teste1">
    <input type="hidden" name='produto2' value="teste2">
    <input type="hidden" name="produto3" value="teste3">
    <input type="hidden" name='produto4' value="teste4">
    <input type="hidden" name="produto5" value="teste5">
<input type="submit" value="enviar">
</form>"
EOD;
 
$dom = new DOMDocument();
$dom->loadHTML($html);
 
$tags = $dom->getElementsByTagName('input');
 
foreach ($tags as $tag) {
       echo $tag->getAttribute('name') . "\n";
}

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