Warning: ocilogon(): _oci_open_session: OCIHandleAlloc OCI_HTYPE_SVCCTX: OCI_INVALID_HANDLE can also be caused by running the webserver with an ORACLE_HOME of a (radically?) different version than PHP was linked against.
I experienced this when I accidently used an Oracle 8 ORACLE_HOME for a PHP that was linked against Oracle 9.