Source

File: wp-includes/pluggable.php

function wp_set_current_user( $id, $name = '' ) {
		global $current_user;

		// If `$id` matches the current user, there is nothing to do.
		if ( isset( $current_user )
		&& ( $current_user instanceof WP_User )
		&& ( $id == $current_user->ID )
		&& ( null !== $id )
		) {
			return $current_user;
		}

		$current_user = new WP_User( $id, $name );

		setup_userdata( $current_user->ID );

		/**
		 * Fires after the current user is set.
		 *
		 * @since 2.0.1
		 */
		do_action( 'set_current_user' );

		return $current_user;
	}