-// Set initial default constants including WP_MEMORY_LIMIT, WP_DEBUG, WP_CONTENT_DIR and WP_CACHE.
-wp_initial_constants( );
+/**
+ * If not already configured, `$blog_id` will default to 1 in a single site
+ * configuration. In multisite, it will be overridden by default in ms-settings.php.
+ *
+ * @global int $blog_id
+ * @since 2.0.0
+ */
+global $blog_id;
+
+// Set initial default constants including WP_MEMORY_LIMIT, WP_MAX_MEMORY_LIMIT, WP_DEBUG, SCRIPT_DEBUG, WP_CONTENT_DIR and WP_CACHE.
+wp_initial_constants();
+
+// Check for the required PHP version and for the MySQL extension or a database drop-in.
+wp_check_php_mysql_versions();