Updates supported PHP versions.

This update addresses the issue raised in [GitHub Issue #12760](https://github.com/protocolbuffers/protobuf/issues/12760).

PiperOrigin-RevId: 532116533

php/README.md

@@ -14,10 +14,14 @@ generation functionality.
 
 ## Requirements
 
-To use PHP runtime library requires:
+Using the PHP runtime library requires:
 
-- C extension: PHP 7.x, 8.0
-- [PHP package](http://php.net/downloads.php): PHP 5.5, 5.6, 7.x, or 8.0.
+-   C extension: PHP 7.x, 8.x
+-   [PHP package](http://php.net/downloads.php): PHP 7.4+
+
+For information on how the support levels for PHP versions will change over
+time, see
+[Supported PHP versions](https://cloud.google.com/php/getting-started/supported-php-versions).
 
 ## Installation