Memory cache is disblead by fefault because it is experemental and isn't tested enough. It can be enabled whit configuartion directive "soap.wsdl_cache = WSDL_CACHE_MEMORY" or "soap.wsdl_cache = WSDL_CACHE_BOTH" or derectly in SoapClent/SoapServer constructor: $ws = new SoapClient($wsdl, array("cache_wsdl"=>WSDL_CACHE_BOTH))
Disk and memory caches use the same TTL value - "soap.wsdl_cache_ttl".
Configuration directive "soap.wsdl_cache_limit" restricts the number of cached wsdl files in memory. Adding new files into full memory cache will delete oldest files from it.
Note that, each PHP process or thread uses it's own memory cache.