by Patrick Pollet.
@lilian
You are missing an array in your paramters forming line
$enrolment = array(
'roleid' => 5,
'userid' => 24,
'courseid' => 69
);
$enrolments = array( $enrolment );
$params = array( 'enrolments' => $enrolments );
it should be
$enrolments = array( array($enrolment ));
Note that a tool to generate proper imbrication of parameters for all WS calls has been submitted here
http://tracker.moodle.org/browse/CONTRIB-3472
Cheers
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