You could run into this error if you have the XDebug extension installed and you execute a lot of requests in callbacks. This error message comes specifically from the XDebug extension. PHP itself does not have a function nesting limit. Change this setting in your php.ini to increase the limit:
xdebug.max_nesting_level = 1000
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There are several things you can do to speed up your client:
This can occur for a number of reasons, but if you are sending PUT, POST, or PATCH requests with an Expect: 100-Continue header, a server that does not support this header will return a 417 response. You can work around this by calling $request->removeHeader('Expect'); after setting the entity body of a request.