function check_comment( $author, $email, $url, $comment, $user_ip, $user_agent, $comment_type ) {
	global $wpdb;

	// If manual moderation is enabled, skip all checks and return false.
	if ( 1 == get_option( 'comment_moderation' ) ) {
		return false;
	}

	/** This filter is documented in wp-includes/comment-template.php */
	$comment = apply_filters( 'comment_text', $comment, null, array() );

	// Check for the number of external links if a max allowed number is set.
	if ( $max_links = get_option( 'comment_max_links' ) ) {
		$num_links = preg_match_all( '/<a [^>]*href/i', $comment, $out );

		/**
		 * Filters the number of links found in a comment.
		 *
		 * @since 3.0.0
		 * @since 4.7.0 Added the `$comment` parameter.
		 *
		 * @param int    $num_links The number of links found.
		 * @param string $url       Comment author's URL. Included in allowed links total.
		 * @param string $comment   Content of the comment.
		 */
		$num_links = apply_filters( 'comment_max_links_url', $num_links, $url, $comment );

		/*
		 * If the number of links in the comment exceeds the allowed amount,
		 * fail the check by returning false.
		 */
		if ( $num_links >= $max_links ) {
			return false;
		}
	}

	$mod_keys = trim( get_option( 'moderation_keys' ) );

	// If moderation 'keys' (keywords) are set, process them.
	if ( ! empty( $mod_keys ) ) {
		$words = explode( "\n", $mod_keys );

		foreach ( (array) $words as $word ) {
			$word = trim( $word );

			// Skip empty lines.
			if ( empty( $word ) ) {
				continue;
			}

			/*
			 * Do some escaping magic so that '#' (number of) characters in the spam
			 * words don't break things:
			 */
			$word = preg_quote( $word, '#' );

			/*
			 * Check the comment fields for moderation keywords. If any are found,
			 * fail the check for the given field by returning false.
			 */
			$pattern = "#$word#i";
			if ( preg_match( $pattern, $author ) ) {
				return false;
			}
			if ( preg_match( $pattern, $email ) ) {
				return false;
			}
			if ( preg_match( $pattern, $url ) ) {
				return false;
			}
			if ( preg_match( $pattern, $comment ) ) {
				return false;
			}
			if ( preg_match( $pattern, $user_ip ) ) {
				return false;
			}
			if ( preg_match( $pattern, $user_agent ) ) {
				return false;
			}
		}
	}

	/*
	 * Check if the option to approve comments by previously-approved authors is enabled.
	 *
	 * If it is enabled, check whether the comment author has a previously-approved comment,
	 * as well as whether there are any moderation keywords (if set) present in the author
	 * email address. If both checks pass, return true. Otherwise, return false.
	 */
	if ( 1 == get_option( 'comment_whitelist' ) ) {
		if ( 'trackback' != $comment_type && 'pingback' != $comment_type && $author != '' && $email != '' ) {
			$comment_user = get_user_by( 'email', wp_unslash( $email ) );
			if ( ! empty( $comment_user->ID ) ) {
				$ok_to_comment = $wpdb->get_var( $wpdb->prepare( "SELECT comment_approved FROM $wpdb->comments WHERE user_id = %d AND comment_approved = '1' LIMIT 1", $comment_user->ID ) );
			} else {
				// expected_slashed ($author, $email)
				$ok_to_comment = $wpdb->get_var( $wpdb->prepare( "SELECT comment_approved FROM $wpdb->comments WHERE comment_author = %s AND comment_author_email = %s and comment_approved = '1' LIMIT 1", $author, $email ) );
			}
			if ( ( 1 == $ok_to_comment ) &&
				( empty( $mod_keys ) || false === strpos( $email, $mod_keys ) ) ) {
					return true;
			} else {
				return false;
			}
		} else {
			return false;
		}
	}
	return true;
}