怎么判断每组数组里有多少个正数和负数

$arr=array(
  array('3','-1','4','2','-2'),
  array('-3','-1','-4','-12','-2','21','13','-16'),
  array('-5','-8','4','2')
);
foreach($arr as $k=>$a) {
  $n = count(array_keys(array_map('sign',$a), -1));
  $r[$k] = array('+' => count($a) - $n, '-' => $n);
}

print_r($r);

function sign($n) {
  return $n/abs($n);
}