PHP基础之运算符4——位运算符

位运算符允许对整型数中指定的位进行求值和操作。

位运算符例子名称结果$a & $bAnd（按位与）将把 $a 和 $b 中都为 1 的位设为 1。$a | $bOr（按位同或）将把 $a 和 $b 中任何一个为 1 的位设为 1。$a ^ $bXor（按位异或）将把 $a 和 $b 中一个为 1 另一个为 0 的位设为 1。~ $aNot（按位取反）将 $a 中为 0 的位设为 1，反之亦然。$a << $bShift left（左移）将 $a 中的位向左移动 $b 次（每一次移动都表示“乘以 2”）。$a >> $bShift right（右移）将 $a 中的位向右移动 $b 次（每一次移动都表示“除以 2”）。

位移在 PHP 中是数学运算。向任何方向移出去的位都被丢弃。左移时右侧以零填充，符号位被移走意味着正负号不被保留。右移时左侧以符号位填充，意味着正负号被保留。

要用括号确保想要的优先级。例如 $a & $b == true 先进行比较再进行按位与；而 ($a & $b) == true 则先进行按位与再进行比较。

要注意数据类型的转换。如果左右参数都是字符串，则位运算符将对字符的 ASCII 值进行操作。PHP 的 ini 设定 error_reporting 使用了按位的值，提供了关闭某个位的真实例子。要显示除了提示级别之外的所有错误，php.ini 中是这样用的：E_ALL & ~E_NOTICE

具体运作方式是先取得 E_ALL 的值：00000000000000000111011111111111再取得 E_NOTICE 的值：00000000000000000000000000001000然后通过 ~ 将其取反：11111111111111111111111111110111最后再用按位与 AND（&）得到两个值中都设定了（为 1）的位：00000000000000000111011111110111

另外一个方法是用按位异或 XOR（^）来取得只在其中一个值中设定了的位：E_ALL ^ E_NOTICEerror_reporting 也可用来演示怎样置位。只显示错误和可恢复错误的方法是：E_ERROR | E_RECOVERABLE_ERROR

也就是将 E_ERROR00000000000000000000000000000001和 E_RECOVERABLE_ERROR00000000000000000001000000000000用按位或 OR（|）运算符来取得在任何一个值中被置位的结果：00000000000000000001000000000001

Example #1 整数的 AND，OR 和 XOR 位运算符

<?php/** Ignore the top section,* it is just formatting to make output clearer.*/$format = ’(%1$2d = %1$04b) = (%2$2d = %2$04b)’. ’ %3$s (%4$2d = %4$04b)’ . 'n';echo <<<EOH---------     ---------  -- ---------result        value      op test---------     ---------  -- ---------EOH;/** Here are the examples.*/$values = array(0, 1, 2, 4, 8);$test = 1 + 4;echo 'n Bitwise AND n';foreach ($values as $value) {$result = $value & $test;printf($format, $result, $value, ’&’, $test);}echo 'n Bitwise Inclusive OR n';foreach ($values as $value) {$result = $value | $test;printf($format, $result, $value, ’|’, $test);}echo 'n Bitwise Exclusive OR (XOR) n';foreach ($values as $value) {$result = $value ^ $test;printf($format, $result, $value, ’^’, $test);}?>

以上例程会输出：

--------- --------- -- --------- resultvalue op test --------- --------- -- --------- Bitwise AND( 0 = 0000) = ( 0 = 0000) & ( 5 = 0101)( 1 = 0001) = ( 1 = 0001) & ( 5 = 0101)( 0 = 0000) = ( 2 = 0010) & ( 5 = 0101)( 4 = 0100) = ( 4 = 0100) & ( 5 = 0101)( 0 = 0000) = ( 8 = 1000) & ( 5 = 0101) Bitwise Inclusive OR( 5 = 0101) = ( 0 = 0000) | ( 5 = 0101)( 5 = 0101) = ( 1 = 0001) | ( 5 = 0101)( 7 = 0111) = ( 2 = 0010) | ( 5 = 0101)( 5 = 0101) = ( 4 = 0100) | ( 5 = 0101)(13 = 1101) = ( 8 = 1000) | ( 5 = 0101) Bitwise Exclusive OR (XOR)( 5 = 0101) = ( 0 = 0000) ^ ( 5 = 0101)( 4 = 0100) = ( 1 = 0001) ^ ( 5 = 0101)( 7 = 0111) = ( 2 = 0010) ^ ( 5 = 0101)( 1 = 0001) = ( 4 = 0100) ^ ( 5 = 0101)(13 = 1101) = ( 8 = 1000) ^ ( 5 = 0101)

Example #2 字符串的 XOR 运算符

<?phpecho 12 ^ 9; // Outputs ’5’echo '12' ^ '9'; // Outputs the Backspace character (ascii 8)// (’1’ (ascii 49)) ^ (’9’ (ascii 57)) = #8echo 'hallo' ^ 'hello'; // Outputs the ascii values #0 #4 #0 #0 #0// ’a’ ^ ’e’ = #4echo 2 ^ '3'; // Outputs 1// 2 ^ ((int)'3') == 1echo '2' ^ 3; // Outputs 1// ((int)'2') ^ 3 == 1?>

Example #3 整数的位移

<?php/** Here are the examples.*/echo 'n--- BIT SHIFT RIGHT ON POSITIVE INTEGERS ---n';$val = 4;$places = 1;$res = $val >> $places;p($res, $val, ’>>’, $places, ’copy of sign bit shifted into left side’);$val = 4;$places = 2;$res = $val >> $places;p($res, $val, ’>>’, $places);$val = 4;$places = 3;$res = $val >> $places;p($res, $val, ’>>’, $places, ’bits shift out right side’);$val = 4;$places = 4;$res = $val >> $places;p($res, $val, ’>>’, $places, ’same result as above; can not shift beyond 0’);echo 'n--- BIT SHIFT RIGHT ON NEGATIVE INTEGERS ---n';$val = -4;$places = 1;$res = $val >> $places;p($res, $val, ’>>’, $places, ’copy of sign bit shifted into left side’);$val = -4;$places = 2;$res = $val >> $places;p($res, $val, ’>>’, $places, ’bits shift out right side’);$val = -4;$places = 3;$res = $val >> $places;p($res, $val, ’>>’, $places, ’same result as above; can not shift beyond -1’);echo 'n--- BIT SHIFT LEFT ON POSITIVE INTEGERS ---n';$val = 4;$places = 1;$res = $val << $places;p($res, $val, ’<<’, $places, ’zeros fill in right side’);$val = 4;$places = (PHP_INT_SIZE * 8) - 4;$res = $val << $places;p($res, $val, ’<<’, $places);$val = 4;$places = (PHP_INT_SIZE * 8) - 3;$res = $val << $places;p($res, $val, ’<<’, $places, ’sign bits get shifted out’);$val = 4;$places = (PHP_INT_SIZE * 8) - 2;$res = $val << $places;p($res, $val, ’<<’, $places, ’bits shift out left side’);echo 'n--- BIT SHIFT LEFT ON NEGATIVE INTEGERS ---n';$val = -4;$places = 1;$res = $val << $places;p($res, $val, ’<<’, $places, ’zeros fill in right side’);$val = -4;$places = (PHP_INT_SIZE * 8) - 3;$res = $val << $places;p($res, $val, ’<<’, $places);$val = -4;$places = (PHP_INT_SIZE * 8) - 2;$res = $val << $places;p($res, $val, ’<<’, $places, ’bits shift out left side, including sign bit’);/** Ignore this bottom section,* it is just formatting to make output clearer.*/function p($res, $val, $op, $places, $note = ’’) {$format = ’%0’ . (PHP_INT_SIZE * 8) . 'bn';printf('Expression: %d = %d %s %dn', $res, $val, $op, $places);echo ' Decimal:n';printf('  val=%dn', $val);printf('  res=%dn', $res);echo ' Binary:n';printf(’  val=’ . $format, $val);printf(’  res=’ . $format, $res);if ($note) {echo ' NOTE: $noten';}echo 'n';}?>

以上例程在 32 位机器上的输出:

--- BIT SHIFT RIGHT ON POSITIVE INTEGERS ---Expression: 2 = 4 >> 1 Decimal: val=4 res=2 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000010 NOTE: copy of sign bit shifted into left sideExpression: 1 = 4 >> 2 Decimal: val=4 res=1 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000001Expression: 0 = 4 >> 3 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: bits shift out right sideExpression: 0 = 4 >> 4 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: same result as above; can not shift beyond 0--- BIT SHIFT RIGHT ON NEGATIVE INTEGERS ---Expression: -2 = -4 >> 1 Decimal: val=-4 res=-2 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111110 NOTE: copy of sign bit shifted into left sideExpression: -1 = -4 >> 2 Decimal: val=-4 res=-1 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111111 NOTE: bits shift out right sideExpression: -1 = -4 >> 3 Decimal: val=-4 res=-1 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111111 NOTE: same result as above; can not shift beyond -1--- BIT SHIFT LEFT ON POSITIVE INTEGERS ---Expression: 8 = 4 << 1 Decimal: val=4 res=8 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000001000 NOTE: zeros fill in right sideExpression: 1073741824 = 4 << 28 Decimal: val=4 res=1073741824 Binary: val=00000000000000000000000000000100 res=01000000000000000000000000000000Expression: -2147483648 = 4 << 29 Decimal: val=4 res=-2147483648 Binary: val=00000000000000000000000000000100 res=10000000000000000000000000000000 NOTE: sign bits get shifted outExpression: 0 = 4 << 30 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: bits shift out left side--- BIT SHIFT LEFT ON NEGATIVE INTEGERS ---Expression: -8 = -4 << 1 Decimal: val=-4 res=-8 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111000 NOTE: zeros fill in right sideExpression: -2147483648 = -4 << 29 Decimal: val=-4 res=-2147483648 Binary: val=11111111111111111111111111111100 res=10000000000000000000000000000000Expression: 0 = -4 << 30 Decimal: val=-4 res=0 Binary: val=11111111111111111111111111111100 res=00000000000000000000000000000000 NOTE: bits shift out left side, including sign bit

以上例程在 64 位机器上的输出:

--- BIT SHIFT RIGHT ON POSITIVE INTEGERS ---Expression: 2 = 4 >> 1 Decimal: val=4 res=2 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000010 NOTE: copy of sign bit shifted into left sideExpression: 1 = 4 >> 2 Decimal: val=4 res=1 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000001Expression: 0 = 4 >> 3 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out right sideExpression: 0 = 4 >> 4 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: same result as above; can not shift beyond 0--- BIT SHIFT RIGHT ON NEGATIVE INTEGERS ---Expression: -2 = -4 >> 1 Decimal: val=-4 res=-2 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111110 NOTE: copy of sign bit shifted into left sideExpression: -1 = -4 >> 2 Decimal: val=-4 res=-1 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111111 NOTE: bits shift out right sideExpression: -1 = -4 >> 3 Decimal: val=-4 res=-1 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111111 NOTE: same result as above; can not shift beyond -1--- BIT SHIFT LEFT ON POSITIVE INTEGERS ---Expression: 8 = 4 << 1 Decimal: val=4 res=8 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000001000 NOTE: zeros fill in right sideExpression: 4611686018427387904 = 4 << 60 Decimal: val=4 res=4611686018427387904 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0100000000000000000000000000000000000000000000000000000000000000Expression: -9223372036854775808 = 4 << 61 Decimal: val=4 res=-9223372036854775808 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=1000000000000000000000000000000000000000000000000000000000000000 NOTE: sign bits get shifted outExpression: 0 = 4 << 62 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out left side--- BIT SHIFT LEFT ON NEGATIVE INTEGERS ---Expression: -8 = -4 << 1 Decimal: val=-4 res=-8 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111000 NOTE: zeros fill in right sideExpression: -9223372036854775808 = -4 << 61 Decimal: val=-4 res=-9223372036854775808 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1000000000000000000000000000000000000000000000000000000000000000Expression: 0 = -4 << 62 Decimal: val=-4 res=0 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out left side, including sign bit

Warning

不要在 32 位系统下向右移超过 32 位。不要在结果可能超过 32 的情况下左移。使用 gmp 扩展对超出 PHP_INT_MAX 的数值来进行位操作。