三维数组的数据处理问题，求大佬来

[_IronBear]

array(8) {
  [0] => array(2) {
    [0] => array(14) {
      ["danName"] => string(6) "甲烷"
      ["standard"] => string(3) "CH4"
      ["danCate"] => string(15) "易燃性气体"
      ["price"] => string(5) "10.00"
      ["unit"] => string(9) "立方米"
      ["stock"] => int(5)
      ["id"] => int(41)
      ["date"] => string(19) "2018-01-12 11:16:23"
      ["operator"] => string(5) "admin"
      ["remark"] => string(18) "甲烷领用测试"
      ["borrower"] => string(5) "aut77"
      ["num"] => int(5)
      ["title"] => string(9) "安环部"
      ["deptId"] => int(3)
    }
    ["sum"] => int(50)
  }
  [1] => array(4) {
    [0] => array(14) {
      ["danName"] => string(6) "硫磺"
      ["standard"] => string(13) "GB/T2449-2006"
      ["danCate"] => string(18) "易燃性固体233"
      ["price"] => string(6) "120.00"
      ["unit"] => string(2) "kg"
      ["stock"] => int(69)
      ["id"] => int(1)
      ["date"] => string(19) "2018-01-15 14:37:29"
      ["operator"] => string(0) ""
      ["remark"] => string(21) "硫磺领用测试233"
      ["borrower"] => string(5) "admin"
      ["num"] => int(4)
      ["title"] => string(9) "行政部"
      ["deptId"] => int(2)
    }
    [1] => array(14) {
      ["danName"] => string(6) "乙烷"
      ["standard"] => string(3) "YW1"
      ["danCate"] => string(15) "易燃性气体"
      ["price"] => string(5) "10.00"
      ["unit"] => string(2) "KG"
      ["stock"] => int(85)
      ["id"] => int(1)
      ["date"] => string(19) "2018-01-12 15:08:04"
      ["operator"] => string(5) "admin"
      ["remark"] => string(21) "乙烷领用测试123"
      ["borrower"] => string(5) "admin"
      ["num"] => int(20)
      ["title"] => string(9) "行政部"
      ["deptId"] => int(2)
    }
    [2] => array(14) {
      ["danName"] => string(6) "乙烷"
      ["standard"] => string(3) "YW1"
      ["danCate"] => string(15) "易燃性气体"
      ["price"] => string(5) "10.00"
      ["unit"] => string(2) "KG"
      ["stock"] => int(85)
      ["id"] => int(1)
      ["date"] => string(19) "2018-01-12 15:08:11"
      ["operator"] => string(5) "admin"
      ["remark"] => string(21) "乙烷领用测试456"
      ["borrower"] => string(5) "admin"
      ["num"] => int(2)
      ["title"] => string(9) "行政部"
      ["deptId"] => int(2)
    }
    ["sum"] => int(700)
  }
  [2] => array(0) {
  }
  [3] => array(0) {
  }
  [4] => array(0) {
  }
  [5] => array(0) {
  }
  [6] => array(0) {
  }
  [7] => array(0) {
  }
}

这个是数据
比如数组第二层中第二个元素，里面有4个数组元素，3个数字键值的1个字母的，其中第二个和第三个里面的danName都是乙烷
怎么样把这两个danName相同的合并起来？然后其他不变，num这个字段是被合并这2个元素中num的和，date用最近那个

我把json数据发出来，求大佬帮个忙

string(1278) "[{"0":{"danName":"\u7532\u70f7","standard":"CH4","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"\u7acb\u65b9\u7c73","stock":5,"id":41,"date":"2018-01-12 11:16:23","operator":"admin","remark":"\u7532\u70f7\u9886\u7528\u6d4b\u8bd5","borrower":"aut77","num":5,"title":"\u5b89\u73af\u90e8","deptId":3},"sum":50},{"0":{"danName":"\u786b\u78fa","standard":"GB\/T2449-2006","danCate":"\u6613\u71c3\u6027\u56fa\u4f53233","price":"120.00","unit":"kg","stock":69,"id":1,"date":"2018-01-15 14:37:29","operator":"","remark":"\u786b\u78fa\u9886\u7528\u6d4b\u8bd5233","borrower":"admin","num":4,"title":"\u884c\u653f\u90e8","deptId":2},"1":{"danName":"\u4e59\u70f7","standard":"YW1","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"KG","stock":85,"id":1,"date":"2018-01-12 15:08:04","operator":"admin","remark":"\u4e59\u70f7\u9886\u7528\u6d4b\u8bd5123","borrower":"admin","num":20,"title":"\u884c\u653f\u90e8","deptId":2},"2":{"danName":"\u4e59\u70f7","standard":"YW1","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"KG","stock":85,"id":1,"date":"2018-01-12 15:08:11","operator":"admin","remark":"\u4e59\u70f7\u9886\u7528\u6d4b\u8bd5456","borrower":"admin","num":2,"title":"\u884c\u653f\u90e8","deptId":2},"sum":700},[],[],[],[],[],[]]"

[qq_34139573]

$str = '[{"0":{"danName":"\u7532\u70f7","standard":"CH4","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"\u7acb\u65b9\u7c73","stock":5,"id":41,"date":"2018-01-12 11:16:23","operator":"admin","remark":"\u7532\u70f7\u9886\u7528\u6d4b\u8bd5","borrower":"aut77","num":5,"title":"\u5b89\u73af\u90e8","deptId":3},"sum":50},{"0":{"danName":"\u786b\u78fa","standard":"GB\/T2449-2006","danCate":"\u6613\u71c3\u6027\u56fa\u4f53233","price":"120.00","unit":"kg","stock":69,"id":1,"date":"2018-01-15 14:37:29","operator":"","remark":"\u786b\u78fa\u9886\u7528\u6d4b\u8bd5233","borrower":"admin","num":4,"title":"\u884c\u653f\u90e8","deptId":2},"1":{"danName":"\u4e59\u70f7","standard":"YW1","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"KG","stock":85,"id":1,"date":"2018-01-12 15:08:04","operator":"admin","remark":"\u4e59\u70f7\u9886\u7528\u6d4b\u8bd5123","borrower":"admin","num":20,"title":"\u884c\u653f\u90e8","deptId":2},"2":{"danName":"\u4e59\u70f7","standard":"YW1","danCate":"\u6613\u71c3\u6027\u6c14\u4f53","price":"10.00","unit":"KG","stock":85,"id":1,"date":"2018-01-12 15:08:11","operator":"admin","remark":"\u4e59\u70f7\u9886\u7528\u6d4b\u8bd5456","borrower":"admin","num":2,"title":"\u884c\u653f\u90e8","deptId":2},"sum":700},[],[],[],[],[],[]]';

$arr = json_decode($str,1);
foreach ($arr as $k => $v) {
    $tmp = array();
    foreach ($v as $kk => $vv) {
        if(is_array($vv)){
            $name = $vv['danName'];
            if(isset($tmp[$name])){
                $tmp[$name]['num'] += $vv['num'];
                $tmp[$name]['date'] = $tmp[$name]['date']>=$vv['date'] ? $tmp[$name]['date'] : $vv['date'];
            }else{
                $tmp[$name] = $vv;
            }
        }else{
            $tmp[$kk] = $vv;
        }
    }

    $arr[$k] = $tmp;
}
print_r($arr);