This code will insert require_once("new_include_file.php"); with a line-break on the second line of whatever file sed happens to process.

# Inserts code starting at line 2 of a file
2irequire_once("new_include_file.php");\n

normally, you would stick this inside of a .sed file. My personal reason for doing this is that it's easier than dealing with shell escaping on the command line.

A corollary is that you RARELY only need to run a single sed command on a particular set of files. You'd rarely do it on a single file outside of testing, otherwise you'd just edit it by hand and not use sed.