WP_Widget_Text::is_legacy_instance() WordPress Method

	public function is_legacy_instance( $instance ) {

		// Legacy mode when not in visual mode.
		if ( isset( $instance['visual'] ) ) {
			return ! $instance['visual'];
		}

		// Or, the widget has been added/updated in 4.8.0 then filter prop is 'content' and it is no longer legacy.
		if ( isset( $instance['filter'] ) && 'content' === $instance['filter'] ) {
			return false;
		}

		// If the text is empty, then nothing is preventing migration to TinyMCE.
		if ( empty( $instance['text'] ) ) {
			return false;
		}

		$wpautop         = ! empty( $instance['filter'] );
		$has_line_breaks = ( false !== strpos( trim( $instance['text'] ), "\n" ) );

		// If auto-paragraphs are not enabled and there are line breaks, then ensure legacy mode.
		if ( ! $wpautop && $has_line_breaks ) {
			return true;
		}

		// If an HTML comment is present, assume legacy mode.
		if ( false !== strpos( $instance['text'], '<!--' ) ) {
			return true;
		}

		// In the rare case that DOMDocument is not available we cannot reliably sniff content and so we assume legacy.
		if ( ! class_exists( 'DOMDocument' ) ) {
			// @codeCoverageIgnoreStart
			return true;
			// @codeCoverageIgnoreEnd
		}

		$doc = new DOMDocument();

		// Suppress warnings generated by loadHTML.
		$errors = libxml_use_internal_errors( true );
		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged
		@$doc->loadHTML(
			sprintf(
				'<!DOCTYPE html><html><head><meta charset="%s"></head><body>%s</body></html>',
				esc_attr( get_bloginfo( 'charset' ) ),
				$instance['text']
			)
		);
		libxml_use_internal_errors( $errors );

		$body = $doc->getElementsByTagName( 'body' )->item( 0 );

		// See $allowedposttags.
		$safe_elements_attributes = array(
			'strong'  => array(),
			'em'      => array(),
			'b'       => array(),
			'i'       => array(),
			'u'       => array(),
			's'       => array(),
			'ul'      => array(),
			'ol'      => array(),
			'li'      => array(),
			'hr'      => array(),
			'abbr'    => array(),
			'acronym' => array(),
			'code'    => array(),
			'dfn'     => array(),
			'a'       => array(
				'href' => true,
			),
			'img'     => array(
				'src' => true,
				'alt' => true,
			),
		);
		$safe_empty_elements      = array( 'img', 'hr', 'iframe' );

		foreach ( $body->getElementsByTagName( '*' ) as $element ) {
			/** @var DOMElement $element */
			$tag_name = strtolower( $element->nodeName );

			// If the element is not safe, then the instance is legacy.
			if ( ! isset( $safe_elements_attributes[ $tag_name ] ) ) {
				return true;
			}

			// If the element is not safely empty and it has empty contents, then legacy mode.
			if ( ! in_array( $tag_name, $safe_empty_elements, true ) && '' === trim( $element->textContent ) ) {
				return true;
			}

			// If an attribute is not recognized as safe, then the instance is legacy.
			foreach ( $element->attributes as $attribute ) {
				/** @var DOMAttr $attribute */
				$attribute_name = strtolower( $attribute->nodeName );

				if ( ! isset( $safe_elements_attributes[ $tag_name ][ $attribute_name ] ) ) {
					return true;
				}
			}
		}

		// Otherwise, the text contains no elements/attributes that TinyMCE could drop, and therefore the widget does not need legacy mode.
		return false;
	}
Uses	Description
wp-includes/formatting.php:esc_attr()	Escaping for HTML attributes.
wp-includes/general-template.php:get_bloginfo()	Retrieves information about the current site.