Question

Haven’t slept in awhile, so I’m probably missing something simple. Basically, I am taking a number and converting it to three characters. Max number of possibilities is 256*256*256 (16777216). I convert it with:

public function s_encode($num) {
  $num = chr($num / 65536).chr($num / 256).chr($num % 256);      
  return bin2hex($num);  
} 

And convert it back with:

public function s_decode($hex) {    
  $a = pack("H*", $hex);
  $b = ord(substr($a, 1, 1));
  $c = ord(substr($a, 2, 1));
  $d = ord(substr($a, 0, 1));   
  return (($d * 65536) + ($b * 256)) + $c;
}

What’s strange, is this actually works. It does what I want it to, but how could it? In the first code, where I convert it to three characters, the second part of the conversion is:

chr($num / 256)

If the number is greater than 65536, this should cause an error, but it doesn’t. If I were to use unpack instead of bin2hex, it will cause an error. bin2hex won’t. Why and how is bin2hex so magical?

Answer 1

chr() only looks at the lowest 8 bits of its input:

echo "'".chr(320)."'";

yields…

'@'

as does…

echo "'".chr(64)."'";

http://ideone.com/65Itz