The full name of PSR is called "PHP Standards Recommendations" (PHP Standards Recommendations). A proposed specification by FIG (PHP Framework Interoperability Group),
This group was initiated by some framework developers in php|tek in 2009, from the first 5 to more than 20 now.

There are many numbers in the "PHP Standard Recommendations", each number represents a different specification .

E.g:
PHP standard recommendation number 1 ( PSR-1 ), the basic coding standard . For example, the name of the class (Class) must be declared as StudlyCaps, the name of the method (Method) must be declared as camelCase.. etc.
The PHP standard recommendation number 2 ( PSR-2 ) is an extension of PSR-1, which regulates the PHP coding style , such as extends or implements must be on the same line as the class name...etc.
PHP standard recommendation number 4 ( PSR-4 ) defines automatic loading ( Autoloader standard ), Composer is also a member of FIG ,
For example, in the current Laravel framework, his composer.json can see the trace of using PSR-4 automatic loading,
Because the automatic loading of PSR-0 has been listed as deprecated (DEPRECATED), but there is still support in composer , if you really want to use it.

In the table below, I list the approved specifications, and of course there are also those in the review and draft stages. If you are interested, please refer to the link on the right: Recommendations (PSRs)

Num	Title	Editor	Coordinator	Sponsors
1	Basic Coding Standard	Paul M. Jones	N/A	N/A
2	Coding Style Guide	Paul M. Jones	N/A	N/A
3	Logger Interface	Jordi Boggiano	N/A	N/A
4	Autoloading Standard	Paul M. Jones	Phil Sturgeon	Larry Garfield
6	Caching Interface	Larry Garfield	Paul Dragoonis	Robert Hafner
7	HTTP Message Interface	Matthew Weier O'Phinney	Beau Simensen	Paul M. Jones
11	Container Interface	Matthieu Napoli, David Negrir	Matthew Weier O'Phinney	Korvin Szanto
13	Hypermedia Links	Larry Garfield	Matthew Weier O'Phinney	Marc Alexander
16	Simple Cache	Paul Dragoonis	Jordi Boggiano	Fabien Potencier

 
Take PSR-7 HTTP Message Interface as an example:
The reqeust and response objects in the Slim framework support the PSR-7 interface, which makes Slim very free to use any PSR-7 implementation.
E.g:
The route of the Slim program can not return the \Slim\Http\Response instance. It can return the \GuzzleHttp\Psr7\CachingStream instance,
Although Slim provides a PSR-7 implementation, we are free to change the default PSR 7 objects in Slim to other third-party implementations.