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!