If the Mantis core APIs trigger an error, the default Mantis error handler is used which generates HTML and break SOAP. Currently the SOAP API avoids such errors by duplicating the checks that are done in the APIs. The proper solution is to provide a SOAP friendly error handler so that when the core API trigger an error, the appropriate soap error is returned to the webservice client.

The following implementation was contributed by Damien Chevalier:
set_error_handler('mciErrorHandler');

function mciErrorHandler($iErrorCode, $sErrorInfo, $sErrorFile, $iErrorLine) {
global $oServer;
switch($sErrorInfo) {
case ERROR_USER_NOT_FOUND : // same custom handling
$oServer->fault('Server', 'Informations de connection incorrect');
break;
default :
$oServer->fault('Server', 'Erreur inconnu : $iErrorCode = "'.$iErrorCode.'", $sErrorInfo = "'.$sErrorInfo.'", $sErrorFile = "'.$sErrorFile.'", $iErrorLine = "'.$iErrorLine.'"');
break;
}

$oServer->send_response();
exit();

}