Source

File: wp-admin/includes/upgrade.php

function wp_install_maybe_enable_pretty_permalinks() {
	global $wp_rewrite;

	// Bail if a permalink structure is already enabled.
	if ( get_option( 'permalink_structure' ) ) {
		return true;
	}

	/*
	 * The Permalink structures to attempt.
	 *
	 * The first is designed for mod_rewrite or nginx rewriting.
	 *
	 * The second is PATHINFO-based permalinks for web server configurations
	 * without a true rewrite module enabled.
	 */
	$permalink_structures = array(
		'/%year%/%monthnum%/%day%/%postname%/',
		'/index.php/%year%/%monthnum%/%day%/%postname%/',
	);

	foreach ( (array) $permalink_structures as $permalink_structure ) {
		$wp_rewrite->set_permalink_structure( $permalink_structure );

		/*
		 * Flush rules with the hard option to force refresh of the web-server's
		 * rewrite config file (e.g. .htaccess or web.config).
		 */
		$wp_rewrite->flush_rules( true );

		$test_url = '';

		// Test against a real WordPress post.
		$first_post = get_page_by_path( sanitize_title( _x( 'hello-world', 'Default post slug' ) ), OBJECT, 'post' );
		if ( $first_post ) {
			$test_url = get_permalink( $first_post->ID );
		}

		/*
		 * Send a request to the site, and check whether
		 * the 'x-pingback' header is returned as expected.
		 *
		 * Uses wp_remote_get() instead of wp_remote_head() because web servers
		 * can block head requests.
		 */
		$response          = wp_remote_get( $test_url, array( 'timeout' => 5 ) );
		$x_pingback_header = wp_remote_retrieve_header( $response, 'x-pingback' );
		$pretty_permalinks = $x_pingback_header && get_bloginfo( 'pingback_url' ) === $x_pingback_header;

		if ( $pretty_permalinks ) {
			return true;
		}
	}

	/*
	 * If it makes it this far, pretty permalinks failed.
	 * Fallback to query-string permalinks.
	 */
	$wp_rewrite->set_permalink_structure( '' );
	$wp_rewrite->flush_rules( true );

	return false;
}