php: how to get associative array key from numeric index?

You don’t. Your array doesn’t have a key [1]. You could:

• Make a new array, which contains the keys:

  $newArray = array_keys($array);
  echo $newArray[0];
  

  But the value “one” is at $newArray[0], not [1].
  A shortcut would be:

  echo current(array_keys($array));
  

• Get the first key of the array:

   reset($array);
   echo key($array);
  

• Get the key corresponding to the value “value”:

  echo array_search('value', $array);
  

This all depends on what it is exactly you want to do. The fact is, [1] doesn’t correspond to “one” any which way you turn it.