To sort an associative array by index values (or keys), use ksort.
ksort($capitals);
An optional parameter (sort_flags) can be passed to ksort to alter the way that it sorts the array. The following code will sort an associative array numerically by index.
ksort($ar, SORT_NUMERIC);
Without this option, the index values ’15, 2, 1′ would be sorted ‘1, 15, 2’ and with the SORT_NUMERIC option, the values would be sorted 1, 2, 15.
