Be aware that wsdl cache may not be compatible across different PHP versions, which may be important to know when you upgrade your PHP on the same system or use a multi-PHP version setup where you run the same scripts from different PHP versions. In other words, when a SOAP request writes a file to the cache and then you make another SOAP request from another PHP version and this cache file is used then *sometimes* you may experience unexpected errors that you will not know where they come from.
I spent hours trying to figure out why I was getting "Encoding: Violation of encoding rules" errors every time I ran some of my SOAP scripts from PHP 7.3 just hours after switching from PHP 7.2. It turned out I needed to clear all cached wsdl files created by PHP 7.2. Downgrading from 7.3 to 7.2 caused the same errors and required clearing the cache, too.
When transitioning to a new PHP version you may turn off the cache or set a very short wsdl_cache_ttl period - if you can't delete the files yourself. If you need your scripts to run on multiple PHP versions with wsdl cache then you'd need to set wsdl_cache_dir that is different for each PHP version.
