You actually don't need to load the SimpleXML object into that json_encode/decode.

You can use that object already and parse whatever values you needed. Having to encode/decode as an array and to have to access values and then converting it into SimpleXML is too much.

$xml = '<?xml version="1.0"?>
<step number="9">
  <s_name>test</s_name>
  <b_sel>12345</b_sel>
  <b_ind>7</b_ind>
</step>';

$xml = simplexml_load_string(step = $xml->xpath('//step');

echo $step[0]->attributes()->number; // 9

echo $xml->s_name, '<br/>', $xml->s_name, '<br/>', $xml->b_ind;

Sample Output

With DOMDocument alone:

$dom = new DOMDocument;
$dom->loadXML(xpath = new DOMXpath($dom);

echo $xpath->evaluate('string(//step/@number)');

Just keep it simple:

$xml = '<?xml version="1.0"?>
<step number="9">
    <s_name>test</s_name>
    <b_sel>12345</b_sel>
    <b_ind>7</b_ind>
</step>';

function convertStepInformationToArray($stepInformation) {
    $dom = new DOMDocument();
    $dom->loadXML($stepInformation);
    $domx = new DOMXPath(entries = $domx->evaluate("//step");
    return $entries;
}

print_r(convertStepInformationToArray($xml));