The MITRE CVE dictionary describes this issue as:
The SOAP parser in PHP before 5.3.23 and 5.4.x before 5.4.13 allows remote attackers to read arbitrary files via a SOAP WSDL file containing an XML external entity declaration in conjunction with an entity reference, related to an XML External Entity (XXE) issue in the soap_xmlParseFile and soap_xmlParseMemory functions. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-1824.
Find out more about CVE-2013-1643 from the
MITRE CVE dictionary dictionary and