How check if a String is a Valid XML with-out Displaying a Warning in PHP

Use libxml_use_internal_errors() to suppress all XML errors, and libxml_get_errors() to iterate over them afterwards.

Simple XML loading string

libxml_use_internal_errors(true);

$doc = simplexml_load_string($xmlstr);
$xml = explode("\n", $xmlstr);

if (!$doc) {
    $errors = libxml_get_errors();

    foreach ($errors as $error) {
        echo display_xml_error($error, $xml);
    }

    libxml_clear_errors();
}