PHP 4, PHP 5, PHP 6. ssl:// & tls:// since PHP 4.3.0 sslv2:// & sslv3:// since PHP 5.0.2
Note: If no transport is specified, tcp:// will be assumed.
Internet Domain sockets expect a port number in addition to a target address. In the case of fsockopen() this is specified in a second parameter and therefore does not impact the formatting of transport URL. With stream_socket_client() and related functions as with traditional URLs however, the port number is specified as a suffix of the transport URL delimited by a colon.
Note: IPv6 numeric addresses with port numbers
In the second example above, while the IPv4 and hostname examples are left untouched apart from the addition of their colon and portnumber, the IPv6 address is wrapped in square brackets: [fe80::1]. This is to distinguish between the colons used in an IPv6 address and the colon used to delimit the portnumber.
The ssl:// and tls:// transports (available only when openssl support is compiled into PHP) are extensions of the tcp:// transport which include SSL encryption. Since PHP 4.3.0 OpenSSL support must be statically compiled into PHP, since PHP 5.0.0 it may be compiled as a module or statically.
ssl:// will attempt to negotiate an SSL V2, or SSL V3 connection depending on the capabilities and preferences of the remote host. sslv2:// and sslv3:// will select the SSL V2 or SSL V3 protocol explicitly.