onvert a URL into embed HTML. Starts by checking the URL against the regex of
	 * the registered embed handlers. If none of the regex matches and it's enabled, then the URL
	 * will be given to the WP_oEmbed class.
	 *
	 * @param array $attr {
	 *     Shortcode attributes. Optional.
	 *
	 *     @type int $width  Width of the embed in pixels.
	 *     @type int $height Height of the embed in pixels.
	 * }
	 * @param string $url The URL attempting to be embedded.
	 * @return string|false The embed HTML on success, otherwise the original URL.
	 *                      `->maybe_make_link()` can return false on failure.
	 */
	public function shortcode( $attr, $url = '' ) {
		$post = get_post();

		if ( empty( $url ) && ! empty( $attr['src'] ) ) {
			$url = $attr['src'];
		}

		$this->last_url = $url;

		if ( empty( $url ) ) {
			$this->last_attr = $attr;
			return '';
		}

		$rawattr = $attr;
		$attr = wp_parse_args( $attr, wp_embed_defaults( $url ) );

		$this->last_attr = $attr;

		// kses converts & into & and we need to undo this
		// See https://core.trac.wordpress.org/ticket/11311
		$url = str_replace( '&', '&', $url );

		// Look for known internal handlers
		ksort( $this->handlers );
		foreach ( $this->handlers as $priority => $handlers ) {
			foreach ( $handlers as $id => $handler ) {
				if ( preg_match( $handler['regex'], $url, $matches ) && is_callable( $handler['callback'] ) ) {
					if ( false !== $return = call_user_func( $handler['callback'], $matches, $attr, $url, $rawattr ) )
						/**
						 * Filters the returned embed handler.
						 *
						 * @since 2.9.0
						 *
						 * @see WP_Embed::shortcode()
						 *
						 * @param mixed  $return The shortcode callback function to call.
						 * @param string $url    The attempted embed URL.
						 * @param array  $attr   An array of shortcode attributes.
						 */
						return apply_filters( 'embed_handler_html', $return, $url, $attr );
				}
			}
		}

		$post_ID = ( ! empty( $post->ID ) ) ? $post->ID : null;

		// Potentially set by WP_Embed::cache_oembed().
		if ( ! empty( $this->post_ID ) ) {
			$post_ID = $this->post_ID;
		}

		// Check for a cached result (stored as custom post or in the post meta).
		$key_suffix    = md5( $url . serialize( $attr ) );
		$cachekey      = '_oembed_' . $key_suffix;
		$cachekey_time = '_oembed_time_' . $key_suffix;

		/**
		 * Filters the oEmbed TTL value (time to live).
		 *
		 * @since 4.0.0
		 *
		 * @param int    $time    Time to live (in seconds).
		 * @param string $url     The attempted embed URL.
		 * @param array  $attr    An array of shortcode attributes.
		 * @param int    $post_ID Post ID.
		 */
		$ttl = apply_filters( 'oembed_ttl', DAY_IN_SECONDS, $url, $attr, $post_ID );

		$cache      = '';
		$cache_time = 0;

		$cached_post_id = $this->find_oembed_post_id( $key_suffix );

		if ( $post_ID ) {
			$cache = get_post_meta( $post_ID, $cachekey, true );
			$cache_time = get_post_meta( $post_ID, $cachekey_time, true );

			if ( ! $cache_time ) {
				$cache_time = 0;
			}
		} elseif ( $cached_post_id ) {
			$cached_post = get_post( $cached_post_id );

			$cache      = $cached_post->post_content;
			$cache_time = strtotime( $cached_post->post_modified_gmt );
		}

		$cached_recently = ( time() - $cache_time ) < $ttl;

		if ( $this->usecache || $cached_recently ) {
			// Failures are cached. Serve one if we're using the cache.
			if ( '{{unknown}}' === $cache ) {
				return $this->maybe_make_link( $url );
			}

			if ( ! empty( $cache ) ) {
				/**
				 * Filters the cached oEmbed HTML.
				 *
				 * @since 2.9.0
				 *
				 * @see WP_Embed::shortcode()
				 *
				 * @param mixed  $cache   The cached HTML result, stored in post meta.
				 * @param string $url     The attempted embed URL.
				 * @param array  $attr    An array of shortcode attributes.
				 * @param int    $post_ID Post ID.
				 */
				return apply_filters( 'embed_oembed_html', $cache, $url, $attr, $post_ID );
			}
		}

		/**
		 * Filters whether to inspect the given URL for discoverable link tags.
		 *
		 * @since 2.9.0
		 * @since 4.4.0 The default value changed to true.
		 *
		 * @see WP_oEmbed::discover()
		 *
		 * @param bool $enable Whether to enable `<link>` tag discovery. Default true.
		 */
		$attr['discover'] = apply_filters( 'embed_oembed_discover', true );

		// Use oEmbed to get the HTML.
		$html = wp_oembed_get( $url, $attr );

		if ( $post_ID ) {
			if ( $html ) {
				update_post_meta( $post_ID, $cachekey, $html );
				update_post_meta( $post_ID, $cachekey_time, time() );
			} elseif ( ! $cache ) {
				update_post_meta( $post_ID, $cachekey, '{{unknown}}' );
			}
		} else {
			$has_kses = false !== has_filter( 'content_save_pre', 'wp_filter_post_kses' );

			if ( $has_kses ) {
				// Prevent KSES from corrupting JSON in post_content.
				kses_remove_filters();
			}

			$insert_post_args = array(
				'post_name' => $key_suffix,
				'post_status' => 'publish',
				'post_type' => 'oembed_cache',
			);

			if ( $html ) {
				if ( $cached_post_id ) {
					wp_update_post( wp_slash( array(
						'ID' => $cached_post_id,
						'post_content' => $html,
					) ) );
				} else {
					wp_insert_post( wp_slash( array_merge(
						$insert_post_args,
						array(
							'post_content' => $html,
						)
					) ) );
				}
			} elseif ( ! $cache ) {
				wp_insert_post( wp_slash( array_merge(
					$insert_post_args,
					array(
						'post_content' => '{{unknown}}',
					)
				) ) );
			}

			if ( $has_kses ) {
				kses_init_filters();
			}
		}

		// If there was a result, return it.
		if ( $html ) {
			/** This filter is documented in wp-includes/class-wp-embed.php */
			return apply_filters( 'embed_oembed_html', $html, $url, $attr, $post_ID );
		}

		// Still unknown
		return $this->maybe_make_link( $url );
	}

	/**
	 * Delete all oEmbed caches. Unused by core as of 4.0.0.
	 *
	 * @param int $post_ID Post ID to delete the caches for.
	 */
	public function delete_oembed_caches( $post_ID ) {
		$post_metas = get_post_custom_keys( $post_ID );
		if ( empty($post_metas) )
			return;

		foreach ( $post_metas as $post_meta_key ) {
			if ( '_oembed_' == substr( $post_meta_key, 0, 8 ) )
				delete_post_meta( $post_ID, $post_meta_key );
		}
	}

	/**
	 * Triggers a caching of all oEmbed results.
	 *
	 * @param int $post_ID Post ID to do the caching for.
	 */
	public function cache_oembed( $post_ID ) {
		$post = get_post( $post_ID );

		$post_types = get_post_types( array( 'show_ui' => true ) );
		/**
		 * Filters the array of post types to cache oEmbed results for.
		 *
		 * @since 2.9.0
		 *
		 * @param array $post_types Array of post types to cache oEmbed results for. Defaults to post types with `show_ui` set to true.
		 */
		if ( empty( $post->ID ) || ! in_array( $post->post_type, apply_filters( 'embed_cache_oembed_types', $post_types ) ) ){
			return;
		}

		// Trigger a caching
		if ( ! empty( $post->post_content ) ) {
			$this->post_ID = $post->ID;
			$this->usecache = false;

			$content = $this->run_shortcode( $post->post_content );
			$this->autoembed( $content );

			$this->usecache = true;
		}
	}

	/**
	 * Passes any unlinked URLs that are on their own line to WP_Embed::shortcode() for potential embedding.
	 *
	 * @see WP_Embed::autoembed_callback()
	 *
	 * @param string $content The content to be searched.
	 * @return string Potentially modified $content.
	 */
	public function autoembed( $content ) {
		// Replace line breaks from all HTML elements with placeholders.
		$content = wp_replace_in_html_tags( $content, array( "\n" => '<!-- wp-line-break -->' ) );

		if ( preg_match( '#(^|\s|>)https?://#i', $content ) ) {
			// Find URLs on their own line.
			$content = preg_replace_callback( '|^(\s*)(https?://[^\s<>"]+)(\s*)$|im', array( $this, 'autoembed_callback' ), $content );
			// Find URLs in their own paragraph.
			$content = preg_replace_callback( '|(<p(?: [^>]*)?>\s*)(https?://[^\s<>"]+)(\s*<\/p>)|i', array( $this, 'autoembed_callback' ), $content );
		}

		// Put the line breaks back.
		return str_replace( '<!-- wp-line-break -->', "\n", $content );
	}

	/**
	 * Callback function for WP_Embed::autoembed().
	 *
	 * @param array $match A regex match array.
	 * @return string The embed HTML on success, otherwise the original URL.
	 */
	public function autoembed_callback( $match ) {
		$oldval = $this->linkifunknown;
		$this->linkifunknown = false;
		$return = $this->shortcode( array(), $match[2] );
		$this->linkifunknown = $oldval;

		return $match[1] . $return . $match[3];
	}

	/**
	 * Conditionally makes a hyperlink based on an internal class variable.
	 *
	 * @param string $url URL to potentially be linked.
	 * @return false|string Linked URL or the original URL. False if 'return_false_on_fail' is true.
	 */
	public function maybe_make_link( $url ) {
		if ( $this->return_false_on_fail ) {
			return false;
		}

		$output = ( $this->linkifunknown ) ? '<a href="' . esc_url($url) . '">' . esc_html($url) . '</a>' : $url;

		/**
		 * Filters the returned, maybe-linked embed URL.
		 *
		 * @since 2.9.0
		 *
		 * @param string $output The linked or original URL.
		 * @param string $url    The original URL.
		 */
		return apply_filters( 'embed_maybe_make_link', $output, $url );
	}

	/**
	 * Find the oEmbed cache post ID for a given cache key.
	 *
	 * @since 4.9.0
	 *
	 * @param string $cache_key oEmbed cache key.
	 * @return int|null Post ID on success, null on failure.
	 */
	public function find_oembed_post_id( $cache_key ) {
		$cache_group    = 'oembed_cache_post';
		$oembed_post_id = wp_cache_get( $cache_key, $cache_group );

		if ( $oembed_post_id && 'oembed_cache' === get_post_type( $oembed_post_id ) ) {
			return $oembed_post_id;
		}

		$oembed_post_query = new WP_Query( array(
			'post_type'              => 'oembed_cache',
			'post_status'            => 'publish',
			'name'                   => $cache_key,
			'posts_per_page'         => 1,
			'no_found_rows'          => true,
			'cache_results'          => true,
			'update_post_meta_cache' => false,
			'update_post_term_cache' => false,
			'lazy_load_term_meta'    => false,
		) );

		if ( ! empty( $oembed_post_query->posts ) ) {
			// Note: 'fields'=>'ids' is not being used in order to cache the post object as it will be needed.
			$oembed_post_id = $oembed_post_query->posts[0]->ID;
			wp_cache_set( $cache_key, $oembed_post_id, $cache_group );

			return $oembed_post_id;
		}

		return null;
	}
}