如何从函数中获取数组键

for example, I have the following code:

$pars = array('foo'=>array(1, 2, 3, 4));
function x($par){
   // how do i get array key from $par, so it will print/return 'foo'
}
x($pars['foo']);

how do i get the array key, when i call function x($pars['foo'])

try like this,

function x($par){
  foreach ($par as $key => $value)
  {
     //you can get key here
   }
}