How do you escape characters in heredoc?
From the bash(1) man page:
If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded.
cat > /etc/nginx/sites-available/default_php <<"END"
Just with a backslash:
cat > /tmp/boeboe <<END
server {
listen 80 default;
server_name _;
root /var/www/\$host';
}
END