PHP：我怎样才能拥有一个具有相同键的数组？

我有两个数组

$projectsIds=[1,2]

和

$usersIds=[1,2,3]

我已经对每个循环进行了如下模拟：

foreach($projectsIds as $projecID){
           foreach($userIds as $userID){
           $sync_array[$projecID] = array('user_id' => $userID);
            }
       }
dd($sync_array);

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           2 => array:1 [    "user_id" => 1  ]
        ]

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           1 => array:1 [    "user_id" => 2  ] 
           1 => array:1 [    "user_id" => 3  ]
           2 => array:1 [    "user_id" => 1  ]
           2 => array:1 [    "user_id" => 2  ]
           2 => array:1 [    "user_id" => 3  ]
        ]

现在我得到这样的结果：

foreach($projectsIds as $projecID){
           foreach($userIds as $userID){
           $sync_array[$projecID] = array('user_id' => $userID);
            }
       }
dd($sync_array);

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           2 => array:1 [    "user_id" => 1  ]
        ]

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           1 => array:1 [    "user_id" => 2  ] 
           1 => array:1 [    "user_id" => 3  ]
           2 => array:1 [    "user_id" => 1  ]
           2 => array:1 [    "user_id" => 2  ]
           2 => array:1 [    "user_id" => 3  ]
        ]

有可能得到如下结果：

foreach($projectsIds as $projecID){
           foreach($userIds as $userID){
           $sync_array[$projecID] = array('user_id' => $userID);
            }
       }
dd($sync_array);

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           2 => array:1 [    "user_id" => 1  ]
        ]

array:2 [  1 => array:1 [    "user_id" => 1  ]  
           1 => array:1 [    "user_id" => 2  ] 
           1 => array:1 [    "user_id" => 3  ]
           2 => array:1 [    "user_id" => 1  ]
           2 => array:1 [    "user_id" => 2  ]
           2 => array:1 [    "user_id" => 3  ]
        ]

您希望创建一个具有相同密钥的两个或多个实例的应用程序，这是根本不可能的

---

密钥将被视为唯一实体

$sync\u array[$projecID][]=array（'user\u id'=>$userID）？数组键是唯一的。因此，如果您请求阵列，例如，
$ar[1]
，应该返回什么？