The general answer to your question is that it depends. And you get to decide by specifying what your "Content-Type" is in the HTTP headers.
A value of "application/x-www-form-urlencoded" means that your POST body will need to be URL encoded just like a GET parameter string. A value of "multipart/form-data" means that you'll be using content delimiters and NOT url encoding the content.
This answer has a much more thorough explanation if you'd like more information.
For an answer specific to the PHP libraries you're using (CURL), you should read the documentation here.
Here's the relevant information:
TRUE to do a regular HTTP POST. This POST is the normal application/x-www-form-urlencoded kind, most commonly used by HTML forms.
The full data to post in a HTTP "POST" operation. To post a file, prepend a filename with @ and use the full path. The filetype can be explicitly specified by following the filename with the type in the format ';type=mimetype'. This parameter can either be passed as a urlencoded string like 'para1=val1¶2=val2&...' or as an array with the field name as key and field data as value. If value is an array, the Content-Type header will be set to multipart/form-data. As of PHP 5.2.0, value must be an array if files are passed to this option with the @ prefix.
~ Answered on 2012-04-03 22:54:23