Prefix private with _.
Be explicit about the type of client.
use Exception so it doesn't have to be qualified.
Remove html from exception message.
Use ternary to reduce noisy code branching.
Use empty to make code a bit simpler to understand.
Use heredoc rather than concatenated strings to simplify syntax.
Resolve style differences with DWS coding standard.
Use !== instead of !=.
Use Dependency Injection to inject the guzzle client.
Fill out test code coverage with a mock guzzle client.