Source

File: wp-admin/includes/class-wp-filesystem-ftpext.php

public function is_dir( $path ) {
		$cwd    = $this->cwd();
		$result = @ftp_chdir( $this->link, trailingslashit( $path ) );

		if ( $result && $path == $this->cwd() || $this->cwd() != $cwd ) {
			@ftp_chdir( $this->link, $cwd );
			return true;
		}

		return false;
	}