If you're array structure will always be multi-dimensional in that exact fashion, then we can iterate through the elements like such:
$fh = fopen('somefile.csv', 'w') or die('Cannot open the file');
for( $i=0; $i<count($arr); $i++ ){
$str = implode( ',', $arr[$i] );
fwrite( $fh, $str );
fwrite( $fh, "\n" );
}
fclose($fh);
That's one way to do it ... you could do it manually but this way is quicker and easier to understand and read.
Then you would manage your headers something what complex857 is doing to spit out the file. You could then delete the file using unlink() if you no longer needed it, or you could leave it on the server if you wished.