Depending on what version of PHP you're using, you may be using PHP sockets (fopen) or Curl. I think you're using fopen and not Curl as you're thinking.
I was using PHP 5.3.3 which used Curl, and upgraded to 5.6 for memory improvements which caused the Splunk library to switch over to sockets with fopen. I discovered that the SSL communication to the Splunk management port stopped working, because PHP 5.6 now requires SSL verification by default.
The Splunk Management port may be using the default self-signed SSL cert.
You can check by using: openssl s_client -connect 192.168.50.35:8089
Fixing it may require you to add a proper SSL cert, or modify the fopen stream context options. The fopen stream context options can be set in the Http.php file, right around here: https://github.com/splunk/splunk-sdk-php/blob/master/Splunk/Http.php#L99-L108
'ssl' => array(
'verify_peer' => false,
'allow_self_signed' => true,
'verify_peer_name' => false,
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