Asynchronous FTP Upload

I have a project in which folders of data will be stored on a web-server and need to be copied over via FTP as needed. Due to a large amount of files, 1 file at a time results in long periods of time for completion. I will need a PHP class/script that can establish multiple FTP connections to a server to asynchronously upload a local folder at a fast rate (multiple file transfers at once). I have a script already completed that accomplishes this (attached). It unfortunately only works on a single FTP connection which provides very slow completion rates for a large amount of files.