HTML string parsing using dom

You have this:

$html = file_get_html($html_str);
$html->load('
<form name="form1" action="parser.php" method="post">
<input type="text" name="parser1">
</form>');

The error message says that $html is not an object. file_get_html() isn't a builtin function but you appear to be using PHP Simple HTML DOM Parser. Its API documentation says that it returns an object but fails to provide additional info. If we look at the source code:

function file_get_html($url, $use_include_path = false, $context=null, $offset = -1, $maxLen=-1, $lowercase = true, $forceTagsClosed=true, $target_charset = DEFAULT_TARGET_CHARSET, $stripRN=true, $defaultBRText=DEFAULT_BR_TEXT, $defaultSpanText=DEFAULT_SPAN_TEXT)
{
    // We DO force the tags to be terminated.
    $dom = new simple_html_dom(null, $lowercase, $forceTagsClosed, $target_charset, $stripRN, $defaultBRText, $defaultSpanText);
    // For sourceforge users: uncomment the next line and comment the retreive_url_contents line 2 lines down if it is not already done.
    $contents = file_get_contents($url, $use_include_path, $context, $offset);
    // Paperg - use our own mechanism for getting the contents as we want to control the timeout.
    //$contents = retrieve_url_contents($url);
    if (empty($contents) || strlen($contents) > MAX_FILE_SIZE)
    {
        return false;
    }
    // The second parameter can force the selectors to all be lowercase.
    $dom->load($contents, $lowercase, $stripRN);
    return $dom;
}

... we can see it returns FALSE some times:

if (empty($contents) || strlen($contents) > MAX_FILE_SIZE)
{
    return false;
}

So I'd dare say that your POST field is either empty or too large. You should really check that before calling ->load().

Update:

The file_get_html() function:

Creates a DOM object from a file or a URL.

I guess what you really want is str_get_html():

Creates a DOM object from a string.

It might help if you actually check if the form is submitted or not. You have to check and re-check if the input is valid or not.

// check if it's a POST request
if($_SERVER['REQUEST_METHOD'] === 'POST') {
    // check if parser1 is not empty
    if(!empty($_POST['parser1'])) {
        $input = $_POST['parser1'];

        if(filter_var($input, FILTER_VALIDATE_URL)) { // looks like an URL
            $html = file_get_html($input); // download URL
        } else { // lets assume it's HTML, because it's not an URL
            $html = str_get_html($input);
        }

        // If something goes wrong here, the input is invalid
        if(!empty($html)) {
             // parse DOM document here
        } else {
            // There is something wrong with the input
        }
    }
}