Why PHP lose item value of array?

When php create the array, set Doctor in index 1 and Boss in index 2, but 2=>'Lynx' cause php overwrite index 2 and set Lynx in it.

You can set it after setted index or use index for it. For example like

$array2 = array("1"=>'Doctor', 2=>'Lynx', 'Boss', 'Lentin', 'Endless');
// or 
$array2 = array("1"=>'Doctor', 2=>'Boss', 3=>'Lynx', 'Lentin', 'Endless');

When $array is being created, 'Boss' will first be stored in index 2 (Array([2] =>'Boss') which is later overwritten by 'Lynx'

Your issue is index keys

$array2 = array("1"=>'Doctor','Boss', 2=>'Lynx', 'Lentin', 'Endless');
print_r ($array2);

This is because, on index 1 it is already doctor, boss will be second, which will be replaced by Lynx which have same index of 2 where boss will be replaced.

I hope I am clear.