## Internet Domain: TCP, UDP, SSL, and TLS

`ssl://`, `tls://`, `sslv2://` & `sslv3://`.

> Note: The `sslv2://` and `sslv3://` transports are obsolete and should not be used. They are documented for backward compatibility only.

> Note: If no transport is specified, `tcp://` will be assumed.

- `127.0.0.1`
- `fe80::1`
- `www.example.com`
- `tcp://127.0.0.1`
- `tcp://fe80::1`
- `tcp://www.example.com`
- `udp://www.example.com`
- `ssl://www.example.com`
- `tls://www.example.com`

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.

- `tcp://127.0.0.1:80`
- `tcp://[fe80::1]:80`
- `tcp://www.example.com:80`

> 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.

`ssl://` will attempt to negotiate a secure SSL/TLS connection depending on the capabilities and preferences of both the client and the remote host. The actual protocols that may be used are determined by the OpenSSL configuration and by any options provided through stream_context_create(), such as `ssl.crypto_method`.

The SSLv2 and SSLv3 protocols are obsolete and insecure. Their use is strongly discouraged and they are no longer enabled by default in modern versions of PHP and OpenSSL.

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