ODBC SQL Server driver on Linux/nginx
I'd say you miss a post for your instance in /etc/freetds.conf. The post should look something like this 

[my_alias]
    host = some_db_host_address
    port = 1433
    tds version = 8.0


Then in your connection string you should use that alias. The host and port part shouldn't be needed as they are picked up from the configuration file. You may have to say which driver to use. This works for me

$conn = odbc_connect("Driver=FreeTDS;DSN=my_alias","user","pass");