wpdb::set_prefix( string $prefix, bool $set_table_names = true )

	public function set_prefix( $prefix, $set_table_names = true ) {

		if ( preg_match( '|[^a-z0-9_]|i', $prefix ) )
			return new WP_Error('invalid_db_prefix', 'Invalid database prefix' );

		$old_prefix = is_multisite() ? '' : $prefix;

		if ( isset( $this->base_prefix ) )
			$old_prefix = $this->base_prefix;

		$this->base_prefix = $prefix;

		if ( $set_table_names ) {
			foreach ( $this->tables( 'global' ) as $table => $prefixed_table )
				$this->$table = $prefixed_table;

			if ( is_multisite() && empty( $this->blogid ) )
				return $old_prefix;

			$this->prefix = $this->get_blog_prefix();

			foreach ( $this->tables( 'blog' ) as $table => $prefixed_table )
				$this->$table = $prefixed_table;

			foreach ( $this->tables( 'old' ) as $table => $prefixed_table )
				$this->$table = $prefixed_table;
		}
		return $old_prefix;
	}