Make sure that mod_file_string is built and loaded and then try the syntax that is described here:
http://wiki.freeswitch.org/wiki/Mod_file_string#Examples

Instead of a comma separated list you can use ! and be sure NOT to put a space after the ! because the function delimits the arguments with spaces. Try something like this:

$conn->execute("play_and_get_digits", 1 1 1000 # /usr/local/freeswitch/en/us/callie/sounds/ivr/please.wav!/usr/local/freeswitch/en/us/callie/sounds/ivr/press-1.wav  /usr/loca/freeswitch/en/us/callie/sounds/ivr/invalid.wav res \\d+);

Let us know how it goes.
-MC