如何在PHP中创建平面JSON对象而不是对象数组

我试图用PHP中的三个数组创建一个平面JSON对象。以下代码的输出是一个包含对象数组的对象：

{
 "Amphibian":[
     {"Frogs":"Green"}
  ],
 "Mammal":[
    {"Bats":"Black"},
    {"Elephants":"Grey"},
    {"Rats":"Black"},
    {"Turtles":"Green"}
  ]
}

然而，这不是我想要的。在循环过程中，是否可以将输出转换为包含平面对象的对象？这是我想要的输出：

{
  "Amphibian":
    {"Frogs":"Green"},
  "Mammal":  
    {"Bats":"Black","Elephants":"Grey","Rats":"Black","Turtles":"Green"}
}

代码如下：

$colors = array("Frogs"=>"Green","Bats"=>"Black","Elephants"=>"Grey","Rats"=>"Black","Turtles"=>"Green");

$allAnimals = array("Frogs","Bats","Elephants","Rats","Turtles");

$group = array("Frogs"=>"Amphibian","Bats"=>"Mammal");

$output = array();

foreach($allAnimals as $key=>$animal){

    if(isset($group[$animal])){

        $groupTitle = $group[$animal];
    }

    $output[$groupTitle][] = array($animal=>$colors[$animal]);

}

print JSON_encode($output);

---

将其修改为

$output[$groupTitle][$animal]=$colors[$animal]

$colors = array("Frogs"=>"Green","Bats"=>"Black","Elephants"=>"Grey","Rats"=>"Black","Turtles"=>"Green");     
$allAnimals = array("Frogs","Bats","Elephants","Rats","Turtles");     
$group = array("Frogs"=>"Amphibian","Bats"=>"Mammal");

$output = array();     
foreach($allAnimals as $key=>$animal){
  if(isset($group[$animal])){
    $groupTitle = $group[$animal];
  }
  $output[$groupTitle][$animal] = $colors[$animal]; //here
}

print JSON_encode($output);

输出：

{
"Amphibian":{"Frogs":"Green"},
"Mammal":{"Bats":"Black","Elephants":"Grey","Rats":"Black","Turtles":"Green"}
}
{
“两栖动物”：{“青蛙”：“绿色”}，
“哺乳动物”：{“蝙蝠”：“黑色”，“大象”：“灰色”，“老鼠”：“黑色”，“海龟”：“绿色”}
}第一个JSON对象和第二个JSON对象之间没有区别。我错过了什么？