[imp] IMAP and SMTP with TLSv1.2

[John Kramer]

Hello list,

I use Horde Webmail 5.2.10. At the moment I can connect to IMAP via port 
143 with tls and SMTP via port 587 with tls. This is part of the 
backends.local.php:
[snip]
     'protocol' => 'imap',
     'port' => 143,
     'secure' => 'tls',
     'smtp' => array(
         'auth' => true,
         'port' => 587,
     ),
[/snip]

How can I upgrade the transport security from TLSv1.0 to TLSv1.2? 
Disabling TLSv1.0 on IMAP or SMTP leads to TLS errors on Horde 
connections. PHP 5.6 and OpenSSL 1.0.1 should be capable of TLSv1.1+. 
Are there any ideas?

I use the following software:
PHP 5.6.14
OpenSSL 1.0.1p
Horde Webmail 5.2.10
Imp 6.2.10
Horde_Imap_Client 2.29.3
Horde_Mail 2.6.2_1
Horde_Smtp 1.9.1

Thanks in advance!

John