Troj/PHPObf-A is a malicious PHP script, that will typically be found on a compromised web server.
The exact purpose of Troj/PHPObf-A may vary, but functionality is likely to include providing remote access to the compromised web server.
SophosLabs have seen malicious PHP scripts detected as Troj/PHPObf-A uploaded to web servers that have been compromised for the purpose of delivering SEO attacks.