Fix the API of the SDK

[stof]

• broken code (wrong typehint, wrong variable) (this makes #3 more important)
• missing return statements in fluent interface
• add missing phpdoc for non-typehinted arguments

[trylika]

Didn't see this PR, related to #8

[romainneutron]

Commit has been merged as of f05446f844669938e752f648e500e49916a06fc0