Fill placeholders in file in single pass

Question

I have a skeleton text file with placeholder strings:

blah blah blah
blah $PLACEHOLDER_1$
blah
$PLACEHOLDER_2$

and so on. Specific \"form\" of

Answer 1

Building on the previous answer, perhaps use an array and compute the sed string?

#!/bin/sh
PLACEHOLDER[0]='string 1'
PLACEHOLDER[1]='multiline 
string 
2'

s="sed -i "
for(( i=0 ; i<${#PLACEHOLDER[*]} ; i++ )) ; do 
    echo ${PLACEHOLDER[$i]}
    s=$s"s/PLACEHOLDER_$i/${PLACEHOLDER[$i]}/g;"
done
echo $s

Seems to fail on the multi-line strings, though.

I don't know how portable Bash arrays might be. Above snippet tested with "GNU bash, version 3.2.17(1)-release (i386-apple-darwin9.0)"