<?php
 
$html = <<<EOD
<form method="post" action="">
    <input type="hidden" name="produto1" value="teste1">
    <input type="hidden" name='produto2' value="teste2">
    <input name="produto3" type="hidden" 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);
 
$inputs = $dom->getElementsByTagName('input');
 
foreach ($inputs as $input) {
	echo $input->getAttribute('name') . "\n";
}

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