I haven’t been able to upgrade Horde for a few months:

Error getting channel info from pear.horde.org: Connection to `ssl://pear.horde.org:443' failed:

Today I finally digged into the issue. The short explanation and fix in my case was:

# php -r "print_r(openssl_get_cert_locations());"
Array
(
    [default_cert_file] => /usr/local/ssl/cert.pem
    [default_cert_file_env] => SSL_CERT_FILE
    [default_cert_dir] => /usr/local/ssl/certs
    [default_cert_dir_env] => SSL_CERT_DIR
    [default_private_dir] => /usr/local/ssl/private
    [default_default_cert_area] => /usr/local/ssl
    [ini_cafile] =>
    [ini_capath] =>
)
# ll /usr/local/ssl/cert.pem
ls: /usr/local/ssl/cert.pem: No such file or directory
# cd /usr/local/ssl
# wget --no-check-certificate https://curl.haxx.se/ca/cacert.pem
# mv cacert.pem cert.pem

Problem solved. Completely unrelated to Horde. Might have been caused by a PHP or OpenSSL upgrade at some point, I guess.