Multisite: Introduce get_main_site_id().

This function can be used to easily get the main site ID of a given network via the optional `$network_id` parameter, which defaults to the current network. The existing `is_main_site()` now uses the new function internally and now accepts an optional `$network_id` parameter as well.

The main purpose of the new function at this point is to ensure that the `WP_Network::$blog_id` property is always set. Magic getters in the class have been adjusted to auto-fill the property when it is accessed and empty. Furthermore the function encapsulates logic that was previously part of `ms_load_current_site_and_network()` and has been replaced with a call to the function now.

Props spacedmonkey, jeremyfelt, johnjamesjacoby, flixos90.
Fixes #29684.


git-svn-id: https://develop.svn.wordpress.org/trunk@41380 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-network.php

@@ -148,9 +148,9 @@ class WP_Network {
 			case 'id':
 				return (int) $this->id;
 			case 'blog_id':
-				return $this->blog_id;
+				return $this->get_main_site_id();
 			case 'site_id':
-				return (int) $this->blog_id;
+				return (int) $this->get_main_site_id();
 		}
 
 		return null;
@@ -201,6 +201,25 @@ class WP_Network {
 		}
 	}
 
+	/**
+	 * Returns the main site ID for the network.
+	 *
+	 * Internal method used by the magic getter for the 'blog_id' and 'site_id'
+	 * properties.
+	 *
+	 * @since 4.9.0
+	 * @see get_main_site_id()
+	 *
+	 * @return string Main site ID as numeric string, for compatibility reasons.
+	 */
+	private function get_main_site_id() {
+		if ( empty( $this->blog_id ) ) {
+			$this->blog_id = (string) get_main_site_id( $this->id );
+		}
+
+		return $this->blog_id;
+	}
+
 	/**
 	 * Set the site name assigned to the network if one has not been populated.
 	 *