src/Core_Command.php
public function multisite_convert( $args, $assoc_args ) {
		if ( is_multisite() ) {
			WP_CLI::error( 'This already is a multisite install.' );
		}

		$assoc_args = self::set_multisite_defaults( $assoc_args );
		if ( ! isset( $assoc_args['title'] ) ) {
			// translators: placeholder is blog name
			$assoc_args['title'] = sprintf( _x( '%s Sites', 'Default network name' ), get_option( 'blogname' ) );
		}

		if ( $this->multisite_convert_( $assoc_args ) ) {
			WP_CLI::success( "Network installed. Don't forget to set up rewrite rules (and a .htaccess file, if using Apache)." );
		}
	}
public function multisite_convert( $args, $assoc_args ) {
		if ( is_multisite() ) {
			WP_CLI::error( 'This already is a multisite install.' );
		}

		$assoc_args = self::set_multisite_defaults( $assoc_args );
		if ( ! isset( $assoc_args['title'] ) ) {
			// translators: placeholder is blog name
			$assoc_args['title'] = sprintf( _x( '%s Sites', 'Default network name' ), get_option( 'blogname' ) );
		}

		if ( $this->multisite_convert_( $assoc_args ) ) {
			WP_CLI::success( "Network installed. Don't forget to set up rewrite rules (and a .htaccess file, if using Apache)." );
		}
	}
Transforms an existing single-site installation into a multisite installation.

Creates the multisite database tables, and adds the multisite constants
to wp-config.php.

For those using WordPress with Apache, remember to update the `.htaccess`
file with the appropriate multisite rewrite rules.

[Review the multisite documentation](https://codex.wordpress.org/Create_A_Network)
for more details about how multisite works.

## OPTIONS

[--title=<network-title>]
: The title of the new network.

[--base=<url-path>]
: Base path after the domain name that each site url will start with.
---
default: /
---

[--subdomains]
: If passed, the network will use subdomains, instead of subdirectories. Doesn't work with 'localhost'.

## EXAMPLES

$ wp core multisite-convert
Set up multisite database tables.
Added multisite constants to wp-config.php.
Success: Network installed. Don't forget to set up rewrite rules.

@subcommand multisite-convert
@alias install-network