+A client application wishing to use PSK ciphersuites for TLSv1.2 and below must
+provide a different callback function. This function will be called when the
+client is sending the ClientKeyExchange message to the server.
+
+The purpose of the callback function is to select the PSK identity and
+the pre-shared key to use during the connection setup phase.
+
+The callback is set using functions SSL_CTX_set_psk_client_callback()
+or SSL_set_psk_client_callback(). The callback function is given the
+connection in parameter B<ssl>, a B<NULL>-terminated PSK identity hint
+sent by the server in parameter B<hint>, a buffer B<identity> of
+length B<max_identity_len> bytes where the resulting
+B<NUL>-terminated identity is to be stored, and a buffer B<psk> of
+length B<max_psk_len> bytes where the resulting pre-shared key is to
+be stored.
+
+The callback for use in TLSv1.2 will also work in TLSv1.3 although it is
+recommended to use SSL_CTX_set_psk_use_session_callback()
+or SSL_set_psk_use_session_callback() for this purpose instead. If TLSv1.3 has
+been negotiated then OpenSSL will first check to see if a callback has been set
+via SSL_CTX_set_psk_use_session_callback() or SSL_set_psk_use_session_callback()
+and it will use that in preference. If no such callback is present then it will
+check to see if a callback has been set via SSL_CTX_set_psk_client_callback() or
+SSL_set_psk_client_callback() and use that. In this case the B<hint> value will
+always be NULL and the handshake digest will default to SHA-256 for any returned
+PSK.
+