把二维数组按某个字段相同的值分组成三维数组

array(6) {
  [0] => array(3) {
    ["date"] => string(10) "2017-01-15"
    ["sid"] => string(2) "14"
    ["amount"] => string(7) "4405.00"
  }
  [1] => array(3) {
    ["date"] => string(10) "2017-01-15"
    ["sid"] => string(2) "13"
    ["amount"] => string(7) "2637.00"
  }
  [2] => array(3) {
    ["date"] => string(10) "2017-01-15"
    ["sid"] => string(2) "12"
    ["amount"] => string(7) "4520.00"
  }
  [3] => array(3) {
    ["date"] => string(10) "2017-01-16"
    ["sid"] => string(2) "14"
    ["amount"] => string(7) "1495.00"
  }
  [4] => array(3) {
    ["date"] => string(10) "2017-01-16"
    ["sid"] => string(2) "13"
    ["amount"] => string(7) "3081.00"
  }
  [5] => array(3) {
    ["date"] => string(10) "2017-01-16"
    ["sid"] => string(2) "12"
    ["amount"] => string(7) "5880.00"
  }
}
原二维数组：

$tmp = [];
foreach($data as $k=>$v){
　　$tmp[$v['date']][] = $v;
}

foreach ($tmp2 as $k => $v) {
　　foreach ($v as $k2 => $v2) {
　　　　$tmp3[$k] += $v2['nums'];
　　}
}

转化后：

array(2) {
  ["2017-01-15"] => array(3) {
    [0] => array(3) {
      ["date"] => string(10) "2017-01-15"
      ["sid"] => string(2) "14"
      ["amount"] => string(7) "4405.00"
    }
    [1] => array(3) {
      ["date"] => string(10) "2017-01-15"
      ["sid"] => string(2) "13"
      ["amount"] => string(7) "2637.00"
    }
    [2] => array(3) {
      ["date"] => string(10) "2017-01-15"
      ["sid"] => string(2) "12"
      ["amount"] => string(7) "4520.00"
    }
  }
  ["2017-01-16"] => array(3) {
    [0] => array(3) {
      ["date"] => string(10) "2017-01-16"
      ["sid"] => string(2) "14"
      ["amount"] => string(7) "1495.00"
    }
    [1] => array(3) {
      ["date"] => string(10) "2017-01-16"
      ["sid"] => string(2) "13"
      ["amount"] => string(7) "3081.00"
    }
    [2] => array(3) {
      ["date"] => string(10) "2017-01-16"
      ["sid"] => string(2) "12"
      ["amount"] => string(7) "5880.00"
    }
  }
}