PHP supports the direct io functions as described in the Posix Standard (Section 6) for performing I/O functions at a lower level than the C-Language stream I/O functions (fopen(), fread(),..).
DIO provides functions and stream wrappers which provide raw and serial low level IO support. The use of the DIO functions should be considered only when direct control of a device is needed. In all other cases, the standard filesystem functions are more than adequate.