I know it's an old issue, but posting in case anyone else stumbles across this. I had a similar issue running Ubuntu 14.04. Same error message. I also couldn't find odbcinst.ini. However, I tried the following (different PDO parameters) and things worked just fine (without any modification of ini files)
$db = new PDO("dblib:host=xxx.xxx.xxx.xxx;dbname=MYDBNAME", "USER", "PASS");
I know it's an old issue, but posting in case anyone else stumbles across this. I had a similar issue running Ubuntu 14.04. Same error message. I also couldn't find odbcinst.ini. However, I tried the following (different PDO parameters) and things worked just fine (without any modification of ini files)
$db = new PDO("dblib:host=xxx.xxx.xxx.xxx;dbname=MYDBNAME", "USER", "PASS");