I found this script. I think I could substitute the PHP version of MultiMarkdown here.

#!/bin/bash

#******************************************#
# This script runs the multimarkdown       #
# script on specified file, redirects      #
# output to html file, and opens resulting #
# file in web browser.                     #
#******************************************#

# gets complete filename of file 
srcfile="`eval echo $1`"	

# gets the filename without .txt
FILENOEXTENSION="`echo $srcfile | sed -e 's/\..*$//'`"	

# Runs perl script and creates .html file
~/MultiMarkdown/bin/multimarkdown2XHTML.pl $srcfile > $FILENOEXTENSION.html	

# opens it up
firefox $FILENOEXTENSION.html