Currently we are using several ways to connect to other servers from PHP code. The easiest way is to use standard cURL php extension. It should be available on majority of servers and it also supports SSL: http://php.net/manual/en/ref.curl.php CURL is considered to be more secure than fopen, snoopy library is buggy and not maintained, there is also the lowlevel socket workaround.

We might use something like getURL() from lib/typo3/class.t3lib_div.php - it has a fallback to fopen(). We could implement proxy user&password (MDL-7921). Sample fsockopen() proxy with user&password code at http://www.php.net/manual/en/function.fsockopen.php#58196

fopen used in:
1/ healthcenter - slashargument test
2/ language import
3/ timezone import
4/ installer

snoopy used in:
1/ magpie - see MDL-7946 (simplepie uses CURL)

curl already required for:
1/ cas

fsocksopen used in (some of them could use the proxy user&password too):
1/ language import - with proxy
2/ auth/fc
3/ enrol/authorize
4/ paypal ipn
5/ ipatlas
6/ phpmailer