Similar to Java Specification Request for Java, it serves the standardization of programming concepts in PHP.
The aim is to enable interoperability of components and to provide a common technical basis for implementation of proven concepts for optimal programming and testing practices.
The PHP-FIG is formed by several PHP frameworks founders.
[1] Each PSR is suggested by members and voted according to an established protocol to act consistently and in line with their agreed upon processes.
The PHP-FIG official website has the PSR documentation that follows the RFC 2119 written by Scott Bradner in March 1997 at Harvard University.