The value attribute of any given node represents the value of the entire branch 

<?php
    tidy_parse_file('test.html');
    tidy_clean_repair();
    
    $head = tidy_get_head();
    $html = tidy_get_html();
    
    $head = htmlentities($head->value);
    $html = htmlentities($html->value);
    echo "<PRE>Head: $head\n\nHTML: $html</PRE>";
?>
Result:
Head: <head>
<title>Hello, World!</title>
</head>

HTML: <html>
<head>
<title>Hello, World!</title>
</head>
<body bgcolor="#ffffff">
<b>A simple HTML document</b>
</body>
</html>