get one Array from Arrays

Question

in controller I have array $checked

array:4 [▼
  0 => "3"
  1 => "4"
  2 => "5"
  3 => "8"
]

from base I want get second array, like this, but cannot:

$proservices = Proservice::whereIn('service_id', $checked)->get('product_id')->toArray();

it gives me 2 arrays:

array:3 [▼
  0 => array:1 [▼
    "product_id" => 14
  ]
  1 => array:1 [▼
    "product_id" => 7
  ]
  2 => array:1 [▼
    "product_id" => 14
  ]
]

help me to get array like this:

array:2 [▼
  0 => "14"
  1 => "7"
]