How can i get Bytes?

  javascript, jquery, php

i want to some actions , but i cant understand what is it.

forexample we have

$m = "{$b}:{$c}:{$d}:0";

echo HMAC_SHA256($a,$m);

now we have to do these :
The first four bytes of the HMAC_SHA256($a,$m) function’s result need to be converted into decimal values (0-255 each). The first byte is divided by 256 , the second byte is divided by 65,256 , the third byte is divided by 16,777,216 , the fourth byte is divided by 4,294,967,296 . The sum of the divisions is an equally distributed value greater than or equal to zero, but smaller than one.

how can i get first four bytes of HMAC_SHA256($a,$m) ?
and convert it to decimal

how can i get final number ?

Source: Ask PHP