The PHP release on 20070504 stated "- Fixed a buffer overflow inside user_filter_factory_create(). (CVE-NO-NAME, by Ilia Alshanetsky)"
The PHP interpreter does not offer a reliable "sandboxed" security layer (as found in, say, a JVM) in which untrusted scripts can be run; any script run by the PHP interpreter must be trusted with the privileges of the interpreter itself. This bug described in CVE-2007-2511 can only be triggered by a script author; since no trust boundary is crossed, this issue is not treated as security-sensitive.