Php DomDocument not finding element by Id

I have written the following php:

$html = "<div id='title'>someTitle</div>";

$docToAdd = new DomDocument();
$docToAdd->validateOnParse = true;
$docToAdd->loadHTML($html);


if($docToAdd->getElementById('title') == null){
    echo "null";
}
else{
  开发者_运维知识库  echo $docToAdd->getElementById('title')->tagName;
}

Why does it echo null when I clearly have just added am element with id 'title'?

Thanks

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If the HTML does not contain a doctype declaration, getElementById will always return null.