requesting ScheduledReports.getReports with format=php returns 'Array' and produces a warning
Notice: Array to string conversion in core/dispatch.php on line 33 Backtrace --> <a href='/0'>#0</a> Piwik\Error::errorHandler(...) called at [core/dispatch.php:33] <a href='/1'>#1</a> require_once(...) called at [index.php:47]
Bug #4115 is i think related. I haven't tested any other API method, but I thunk it is any method, that returns multidimensional arrays.
The PHP renderer is for internal use and outputs an in-memory PHP array, not serialized PHP. To output serialized data, append &serialize=1 to your query string.