note 99589 added to language.operators.bitwise

If, like me, you've never thought about how PHP deals with binary, the output of the bitwise NOT may confuse you. For instance, this: $bin = 2; $notbin = ~$bin; echo "Bin: " . decbin($bin) . " !bin: " . decbin($notbin) . "\n"; returns this: Bin: 10 !bin: 1111111111111111111111111111111111111111111111111111111111111101 The reason is that all binary numbers are treated as 32 bits, even if you've manually entered less. In order to get the result I expected (01), it was necessary to AND the result with the number of bits I wanted: in this case, 2 (the number 3, in decimal). Be aware that all return values will have zeros removed from the left until they reach a bit that is set to 1. Continuing the above example, the following: $notbin_2 = ~$bin & '3'; echo "!bin & 3: " . decbin($notbin_2) . "\n"; returns this: !bin & 3: 1 Note that the actual value was a string of 31 zeros followed by a 1, but the zeros were not shown. This is probably a good thing. Furthermore, the NOT operator uses two's complement, which means the number you get may be even stranger than you expect: using two's complement means that ~2 == -3. There are plenty of good explanations of two's complement online, so I won't go into that question here. If what you want is just to reverse a string of bits without any interpretation, you can use a function like this: function bitnot($bin) { $not = ""; for ($i = 0; $i < strlen($bin); $i++) { if($bin[$i] == 0) { $not .= '1'; } if($bin[$i] == 1) { $not .= '0'; } } return $not; } It takes a binary string of any length, reverses the bits, and returns the new string. You can then treat it as a binary number, use bindec() to turn it into a decimal, or whatever you want. I hope this helps someone as much as it would have helped me a week ago! ---- Server IP: 92.48.119.85 Probable Submitter: 128.119.39.171 ---- Manual Page -- http://www.php.net/manual/en/language.operators.bitwise.php Edit -- https://master.php.net/note/edit/99589 Del: integrated -- https://master.php.net/note/delete/99589/integrated Del: useless -- https://master.php.net/note/delete/99589/useless Del: bad code -- https://master.php.net/note/delete/99589/bad+code Del: spam -- https://master.php.net/note/delete/99589/spam Del: non-english -- https://master.php.net/note/delete/99589/non-english Del: in docs -- https://master.php.net/note/delete/99589/in+docs Del: other reasons-- https://master.php.net/note/delete/99589 Reject -- https://master.php.net/note/reject/99589 Search -- https://master.php.net/manage/user-notes.php