SED: insert text after the last line?

The simplest way is:

sed -i -e '$aTEXTTOEND' filename

How it works

$ matches the last line (it's a normal sed address; 4aTEXTTOEND would insert after the fourth line), a is the append command, and TEXTTOEND is what to append, filename is the file where the TEXTTOEND will be inserted

No need to use sed in that case. How about

echo "<?php" >> file

Assuming you want to put the ending php tag to the files, then

sed -i '$s/$/\n?>/' file

should do the trick