PHP is not only available for many different operating systems, but it also offers native interfaces to a range of different Server APIs, or SAPIs in PHP lingo. The most common PHP SAPI is the Apache 1.3 module; others are CGI, CLI, the IIS filter, the embeddable version of PHP,and so on.

Some SAPIs offer PHP functions that are available only in that SAPI.
For example, the Apache 1.3 SAPI offers a function called apache_note() to
pass information to other Apache modules.

Table below shows some SAPI-specific functions.