Bitwise operations on boolean values

Question

From my knowledge, bitwise operators perform a check on all corresponding bits, as in:

echo 64 | 32;   //prints 96
echo 'a' & 'b'; //prints `

While conditional && and || operators perform operations on boolean values:

echo (int)(true && false); //prints: 0
echo (int)(true || false); //prints: 1

When I (in my head) want to predict the result of a bitwise operation, I first convert the values into their binary representation (which depends on the datatype). After this, I compare it bit-for-bit, and convert the result into the suitable output type (which I suppose is determined by the operands). Although at one point, I tried doing the same with boolean values, which (from my knowledge) only consists of one bit in memory, making true corresponding to 1₂, and making false corresponding to 0₂ (in binary). Performing bitwise operations on these values should therefore produce a similar result as with && and ||, right? To show you what I mean:

true & false    =>      1₂ & 0₂      =>      0₂     =>     false
true | false    =>      1₂ | 0₂      =>      1₂     =>     true
~true           =>      ~1₂          =>      0₂     =>     false

(Not including xor, as there's no corresponding conditional boolean operator.)

To me, it looks like the behaviour should be really equivalent to conditional operators:

true && false   =>      false
true || false   =>      true
!true           =>      false

So therefore, I set this code up to test it:

    echo "true AND false: " . ((true && false) ? "1" : "0") . "<br />\n";
    echo "true OR false: " . ((true || false) ? "1" : "0") . "<br />\n";
    echo "NOT true: " . ((!true) ? "1" : "0") . "<br />\n";

    echo "<br />\n";

    echo "true BITAND false: " . ((true & false) ? "1" : "0") . "<br />\n";
    echo "true BITOR false: " . ((true | false) ? "1" : "0") . "<br />\n";
    echo "BITNOT true: " . ((~true) ? "1" : "0") . "<br />\n";

It gave me the following output:

true AND false: 0
true OR false: 1
NOT true: 0

true BITAND false: 0
true BITOR false: 1

Fatal error: Unsupported operand types in C:\Abyss Web Server\htdocs\handler.php on line 21

So from this, I have two questions:

1. What's the point with && and ||, if we (as it seems) can use & and | on boolean values instead?
2. Why can't I do ~true (or with other words, why aren't boolean values supported)? To me, it sounds logical that ~true returns false.

I did come up with one thing, being that && and || will (sometimes) cast the values into bool and thereafter return the correct results, if we (by mistake) would happen to pass a value that's not of type bool. But to solve that, can't we just do a cast first? Such as:

if ((bool)$foo & (bool)$bar) { ...

It feels like I'm missing a major piece here that changes everything... But just in case I didn't, I included as much info as I could. Could someone make me understand this a bit better, by answering my two questions? I'm pretty confused at this point, and I've been thinking about this for quite some time.

Answer 1

Answer 1

Parts of a boolean expressions (||, &&, !, ...) are only evaluated if needed (from left to right):

  if ($a | func()) { } // func is always called
  if ($a || func()) { } // func is not called if $a is true,
                        // because expression is true whatever func will return
  if ($a && func()) { } // func is not called if $a is false,
                        // because expression is false whatever func will return
  func() || exit(); // exit() will be called if func() returns false

Take a look at the documentation: http://php.net/manual/en/language.operators.logical.php

Answer 2

~true seems not to be meaningful: true is 0x00...01 and ~true will be 0xff...fe and not false 0x000...0:

var_dump(~((int)true)); // prints: int(-2)
echo dechex(~((int)true)); // prints: fffffffffffffffe

Use !-operator instead:

var_dump(!true); // prints: bool(false)

Résumé

Use bitwise operators only if you need to change bits.