To find what encoding a given HTTP POST request uses is easy -- passing an array to CURLOPT_POSTFIELDS results in multipart/form-data:
<?php
curl_setopt(CURLOPT_POSTFIELDS, array('field1' => 'value'));
?>
Passing a URL-encoded string will result in application/x-www-form-urlencoded:
<?php
curl_setopt(CURLOPT_POSTFIELDS, array('field1=value&field2=value2'));
?>
I ran across this when integrating with both a warehouse system and an email system; neither would accept multipart/form-data, but both happily accepted application/x-www-form-urlencoded.
