sed error "unescaped newline inside substitute pattern"

[David Banning]

Here is the php line that gives the error;

cat start_text | sed "s/--maintext--/$test/" > endtext

give error;

sed: 1: "s/--maintext--/ Comment ...": unescaped newline inside substitute pattern

where $test contains customer input from a website form

There is something about the content of the text within the variable $test that is causing the error.

Any pointers would be helpful.