how to extract a tag with a class name's innerhtml

I need to get the innerHTML of the maintext class name of a table tag.
If I use:

$dom = new SimpleXMLElement(file_get_contents($file));//line 87
$tables = $dom->xpath('//table[@class="maintext"]'); 
print_r($tables);

PHP:

I get:

On the other hand if I use:
DOMElement

//SimpleXMLElement
$dom = new DOMElement(file_get_contents($file)); // line 88
$table = $dom->xpath('//table[@class="maintext"]');  
print_r($table);

PHP:

I get: