I wanted to print to a specific tray of my printer, a Dell 3110cn.  Some printers allow you to specify in their config what tray will be used as a default tray, but my printer doesn't.  Googling told me that printing to a specific tray is not a function of PostScript but rather a function of my printer itself.  So, I had to find the function for my specific printer.  I located a PPD file for my printer.  Purely by luck (I searched through the file for the word 'tray') I found the following:
*InputSlot 2ndTray-H/Tray 2 (500 Sheets): "
  3 dict begin
  /MediaPosition 1 def
  /DeferredMediaSelection true def
  /TraySwitch false def
  currentdict end setpagedevice
" *End
  
I saved this as a file named Dell.ps, without the first and last lines, and with a blank line at the end.  Then I used the following command in my program:
<?php ps_include_file($ps, 'Dell.ps'); ?>
It worked!