Contains the Oracle instance to connect to. It can be an » Easy Connect string, or a Connect Name from the tnsnames.ora file, or the name of a local Oracle instance.
If not specified, PHP uses environment variables such as TWO_TASK (on Linux) or LOCAL (on Windows) and ORACLE_SID to determine the Oracle instance to connect to.
To use the Easy Connect naming method, PHP must be linked with Oracle 10g or greater Client libraries. The Easy Connect string for Oracle 10g is of the form: [//]host_name[:port][/service_name]. With Oracle 11g, the syntax is: [//]host_name[:port][/service_name][:server_type][/instance_name]. Service names can be found by running the Oracle utility lsnrctl status on the database server machine.
The tnsnames.ora file can be in the Oracle Net search path, which includes $ORACLE_HOME/network/admin and /etc. Alternatively set TNS_ADMIN so that $TNS_ADMIN/tnsnames.ora is read. Make sure the web daemon has read access to the file.
Simply it is the "HOST".