How to create a web-based PHP that can print to multiple printer(when idle) using silent printing The print job connect to the database. When there is an idle printer, the print job directly goes to it. It need to connect with print server too. 
How do i start? Can someone help me...
Thank you.
I'd recommend starting with connecting your printer(s) to OctoPi. From there, try to find either an API or way to create your own plugin. Then just set up a workstation that will manage your print queue and delegate the prints using some form of web request.

I personally don't recommend creating your own slicing engine. If you want to do everything directly from "the workstation" try looking into Skeinforge (most popular from ReplicatorG slicer), Cura (I believe from Ultimaker), and/or Miracle Grue (from MakerWare) as your slicing engine. You can most documentation/source on GitHub.

In the meantime, try to narrow down the general sense of how you want to accomplish this and edit your question. As a reminder for more technical questions regarding code, please use Stack Overflow. For example, "How to create/use REST request?"