Array indexing rules ?

Question

Is it necessary to have the first index of an array as 0 or do we have some way by which we can assign first array index to be some random number e. g first index as 7 and next element automatically becomes at index 8 and so on..

Answer 1

first element index is 0 and it's goes upto n-1 index
where n is the no. of elements in an array

Answer 2

It's not necessary, you can type:
$ar = array(); // empty array
$ar[7] = 'a'; // index starts from 7
$ar[] = 'b'; // index = 8
$ar[] = 'c'; // index = 9
or equivalent:
$ar = array(7=>'a', 'b', 'c');