PHP fixes

[cknave]

• add ext-json to the composer requirements
• don't use the nonexistent Client $base_uri
• return associative arrays from json_decode

I also wanted to reexamine docblocks for functions. I found this error because my IDE raised a warning when I added a type hint. If they're still too big, single line type hints also exist (e.g. /* @var Client $client */)