It appears that fsockopen prior to php5 did not need the unix:// qualifier when opening a unix domain socket:
php4: fsockopen("/tmp/mysocket"......);
php5: fsockopen("unix:///tmp/mysocket"......);
This caught me out when upgrading.
   unix:// and
   udg://.
  
unix:///tmp/mysockudg:///tmp/mysock
   unix:// provides access to a socket stream
   connection in the Unix domain. udg:// provides
   an alternate transport to a Unix domain socket using the user datagram
   protocol.
  
   Unix domain sockets, unlike Internet domain sockets, do not expect
   a port number.  In the case of fsockopen() the
   portno parameter should be set to 0.
  
Nota: Unix domain sockets are not supported on Windows.
It appears that fsockopen prior to php5 did not need the unix:// qualifier when opening a unix domain socket:
php4: fsockopen("/tmp/mysocket"......);
php5: fsockopen("unix:///tmp/mysocket"......);
This caught me out when upgrading.