gistfile1.php

<?php
$code = '';
if ( isset( $instance[ 'code' ] ) ) {
	$code = $instance[ 'code' ];
	// catch PHP tags
	if ( strpos( $code, '<?' ) !== false ) {
		echo '<div class="error"><p>Embed Code error: You can\'t include PHP code here, it will not work, and will be shown on the frontend. Please remove the PHP code and contact a developer. PHP code is surrounded by &lt;?php and ?&gt;</p></div>';
	}
	$dom = new \DOMDocument();
	libxml_use_internal_errors( true );
	$dom->loadHtml( $code );
	$errors = libxml_get_errors();

	foreach ( $errors as $error ) {
		$return = '<strong>Issue found in the embed code:</strong><br>';
		switch ( $error->level ) {
			case LIBXML_ERR_WARNING:
				$return .= "Warning $error->code: ";
				break;
			case LIBXML_ERR_ERROR:
				$return .= "Error $error->code: ";
				break;
			case LIBXML_ERR_FATAL:
				$return .= "Fatal Error $error->code: ";
				break;
		}

		$return .= trim( $error->message ) .
			"<br>  Line: $error->line" .
			",  Column: $error->column";

		echo '<div class="error"><p>'.wp_kses_post( $return ).'</p></div>';
	}
	libxml_clear_errors();

}