I'm not entirely sure what you're trying to achieve with $string, but providing it's correct:
<?
$flights = PIREPData::getLastReports($userinfo->pilotid, '10');
$string = "";
if(!$flights){
$string = 'No flights have been filed!';
} else {
foreach($flights as $flight)
{
$string = $string.$flight->depicao.'+-+'.$flight->arricao.',+';
}
}
?>
Your string will end up looking like this, for example: "EGKK+-+KJFK,+KJFK+-+EGKK,+EGKK+-+KMIA,+KMIA+-+EGKK,+" and so on...
If that's what you intended?