MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix G: Driver Guidelines for Backward Compatibility

SQLAllocEnv Mapping

When an application calls SQLAllocEnv through an ODBC 3.x driver, the call to SQLAllocEnv(phenv) is mapped to SQLAllocHandle as follows:

1. The Driver Manager allocates an environment handle and returns it to the application. The Driver Manager calls SQLSetEnvAttr to set the SQL_ATTR_ODBC_VERSION environment attribute to SQL_OV_ODBC2.
   
   
2. When the application establishes the first connection to a driver, the Driver Manager calls

   SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, OutputHandlePtr)
   

   in the driver with OutputHandlePtr set to phenv.