PHP无效字符错误

我得到这个错误时运行这段代码:Fatal error: Uncaught exception 'DOMException' with message 'Invalid Character Error' in test.php:29 Stack trace: #0 test.php(29): DOMDocument->createElement('1OhmStable', 'a') #1 {main} thrown in test.php on line 29

来自原始XML文件的节点确实包含无效字符，但是当我从节点中剥离无效字符时，应该创建节点。我需要对原始XML文档进行什么类型的编码?我需要解码saveXML吗?

function __cleanData($c) 
{
    return preg_replace("/[^A-Za-z0-9]/", "",$c);
}
$xml = new DOMDocument('1.0', 'UTF-8');
$xml->load('test.xml');    
$xml->formatOutput = true; 
$append = array();
foreach ($xml->getElementsByTagName('product') as $product ) 
    {
        foreach($product->getElementsByTagName('name') as $name ) 
        {
            $append[] = $name;
        }
                foreach ($append as $a)  
                {
                    $nodeName = __cleanData($a->textContent);
                        $element = $xml->createElement(htmlentities($nodeName) , 'a');
                }
        $product->removeChild($xml->getElementsByTagName('details')->item(0));
        $product->appendChild($element);
    }
$result = $xml->saveXML();
$file = "data.xml";
file_put_contents($file,$result);

原始XML是这样的:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="/v1/xsl/xml_pretty_printer.xsl" type="text/xsl"?>
<products>
<product>
<modelNumber>M100</modelNumber>
<itemId>1553725</itemId>
<details>
  <detail>
    <name>1 Ohm Stable</name>
    <value>600 x 1</value>
  </detail>
 </details>
</product>
 </products>

新文档应该是这样的:

 <?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="/v1/xsl/xml_pretty_printer.xsl" type="text/xsl"?>
<products>
<product>
<modelNumber>M100</modelNumber>
<itemId>1553725</itemId>
  <1 Ohm Stable>
  </1 Ohm Stable>
  </product>
 </products>