Description:
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To connect to MySQL through an ssh tunnel, we now have to have a process that opens that tunnel and then close that process when it is not necessary.

It would be much simpler if you could open a socket stream and when opening the connection with MySQL, specify that you do it through that socket stream.

Test script:
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$scp = ssh2_connect($host, 22)
ssh2_auth_pubkey_file($scp, $user, $pub_key, $pem_key, $key_pwd);
$tunnel = ssh2_tunnel($scp, 'localhost', 3306);
$db = new mysqli(null, 'root', null, null, null, $tunnel);