Source

File: wp-admin/includes/class-wp-upgrader.php

public function maintenance_mode( $enable = false ) {
		global $wp_filesystem;
		$file = $wp_filesystem->abspath() . '.maintenance';
		if ( $enable ) {
			$this->skin->feedback( 'maintenance_start' );
			// Create maintenance file to signal that we are upgrading.
			$maintenance_string = '<?php $upgrading = ' . time() . '; ?>';
			$wp_filesystem->delete( $file );
			$wp_filesystem->put_contents( $file, $maintenance_string, FS_CHMOD_FILE );
		} elseif ( ! $enable && $wp_filesystem->exists( $file ) ) {
			$this->skin->feedback( 'maintenance_end' );
			$wp_filesystem->delete( $file );
		}
	}