If you need to save the output to a variable (e.g. for use within a framework) you can write to a temporary memory-wrapper and retrieve it's contents:
<?php
// output up to 5MB is kept in memory, if it becomes bigger it will automatically be written to a temporary file
$csv = fopen('php://temp/maxmemory:'. (5*1024*1024), 'r+');
fputcsv($csv, array('blah','blah'));
rewind($csv);
// put it all in a variable
$output = stream_get_contents($csv);
?>