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.