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