The following adds a line to /etc/profile to export PATH after startup.
( /my/subFolder is added to the current PATHS )
sudo sed -i 's/export PATH/PATH=$PATH:\/my\/subFolder\nexport PATH/g' /etc/profile
Keep in mind that Raspbian is Debian based. You may want to have a look at how it is done in Debian (and similarly can be done in Raspbian). Something you may need to look at also is runLevel you're running (i.e. singleUser, multiUser or GUI) - but the above may be enough.
https://wiki.debian.org/EnvironmentVariables
~
works in the .profile file. Better try the full path, so/home/pi/.composer/vendor/bin
$HOME