After reading KoKos' post above, I thought that the code I posted right before his should do what he wanted. However, my original post neglected to note the little "Tip" in the documentation above, about a single element replacement.
If one changes the lines in my code above that says:
<?php
if(is_array($replacement))
foreach($replacement as $r_key=>$r_value)
$new_array[$r_key]=$r_value;
?>
to instead say:
<?php
if(is_string($replacement))
$new_array[$key]=$replacement;
elseif(is_array($replacement))
foreach($replacement as $r_key=>$r_value)
$new_array[$r_key]=$r_value;
?>
that will solve the problem.
Sorry for the omission.
