If number formed by last four digits is divisible by 16.
$ last = $ str [ $ n - 1 ] - '0' ; $ second_last = $ str [ $ n - 2 ] - '0' ; $ third_last = $ str [ $ n - 3 ] - '0' ; $ fourth_last = $ str [ $ n - 4 ] - '0' ; return ( ( $ fourth_last * 1000 + $ third_last * 100 + $ second_last * 10 + $ last ) % 16 == 0 ) ; }