PHP has ssh2 stream wrappers (disabled by default), so you can use sftp connections with any function that supports stream wrappers by using ssh2.sftp://
for protocol, e.g.
file_get_contents('ssh2.sftp://user:[email protected]:22/path/to/filename');
or – when also using the ssh2 extension
$connection = ssh2_connect('shell.example.com', 22);
ssh2_auth_password($connection, 'username', 'password');
$sftp = ssh2_sftp($connection);
$stream = fopen("ssh2.sftp://$sftp/path/to/file", 'r');
See http://php.net/manual/en/wrappers.ssh2.php
On a side note, there is also quite a bunch of questions about this topic already: