将SQL查询返回的结果按PHP中的特定列进行分组

Question

我有一个在PHP中运行的查询，该查询获取一些MySQL数据并将其作为对象返回。我将数据分组，但是它在对象中返回结果，每个组都是对象属性。我希望结果以数组形式返回

我希望将$ data变量作为由service_type分组的数组返回，但它是一个对象。

从mysql采样数据

id, service_name, service_type
1, 'oil change', 'fluids'
2, 'tire rotation', 'tires';    
3, 'power steeling fluid', 'fluids'

public function fetchAllServices() {
        $q = $this->db->query("SELECT id, service_name, service_type FROM services ORDER BY service_type, id");
        if ($this->db->affected_rows() > 0) {
            foreach ($q->result() as $a) {
                $data[] = $a;
            }
        } else {
            error_log('No Services Found.)');
            return false;
        }


        foreach( $data as $key => $service  ){  
            $categories[$service->service_type][] = $service;
            $service->service_name = str_replace('-', ' ', $service->service_name );
        }

        $data = $categories ? $categories : $data;
        return $data;
    }

前端javascript上的预期结果

$data: [
  0: {
    category: 'fluids'
    results: [
        {id: 1, service_name: 'oil change', service_type: 'fluids'}
        {id: 3, service_name: 'power steering fluid', service_type: 'fluids'}
    ]
  }
  1: {
    category: 'tires'
    results: [
        {id: 2, service_name: 'tire rotation', service_type: 'tires'}
    ]
  }
];

当前结果：

$data: {
    'fluids': [
        {id: 1, service_name: 'oil change', service_type: 'fluids'}
        {id: 3, service_name: 'power steering fluid', service_type: 'fluids'}
    ]
    'tires': [
        {id: 2, service_name: 'tire rotation', service_type: 'tires'}
    ]

};