if you want to do a GET request with additional body data it will become tricky not to implicitly change the request to a POST, like many notes below correctly state.
So to do the analogy of command line's
curl -XGET 'http://example.org?foo=bar' -d '<baz>some additional data</baz>'
in PHP you'll do, besides your other necessary stuff,
<?php
curl_setopt($curlHandle, CURLOPT_CUSTOMREQUEST, 'GET');
curl_setopt($curlHandle, CURLOPT_POSTFIELDS, '<baz>some additional data</baz>');
?>
during my experiments, every other "similar" way, like e.g. CURLOPT_HTTPGET, didn't send the additional data or fell into POST.