IO may be blocking/non-blocking and synchronous/asynchronous. POSIX API provides synchronous blocking API (e.g. classic read, write, send, recv calls), synchronous non-blocking API (same functions, file descriptors opened with
O_NONBLOCK flag and IO-multiplexing calls) and asynchonous API (functions starting with
Synchronous API is usually used with "one thread/process per fd" style. This is dreadful for resources. Non-blocking API allows to operate with a set of fds in one thread.
To build a dynamic and interactive PHP program, it is useful to output variables and their values. The PHP language allows for multiple methods of value output. This topic covers the standard methods of printing a value in PHP and where these methods can be used.