Version 1.2 or newer of the libssh2 library is
required, although it’s possible newer releases of pecl/ssh2 may
require newer versions (see the release notes).
The ssh2_auth_agent() function will only be
available with libssh >= 1.2.3.
support for stream_set_timeout() to channel streams will
only be available with libssh >= 1.2.9.
Libssh2 comes in two flavours: gcrypt or openssl.
Some linux distributions build libssh2 with the gcrypt library,
some use openssl. Libssh2 has some issues when compiled with
gcrypt, please use openssl.