Prefix bitwise operators

Bitwise operators are like logical operators but executed per bit rather than per boolean value.

// bitwise NOT ~: sets all unset bits and unsets all set bits
printf("%'06b", ~0b110110); // 001001

Bitmask-bitmask operators

Bitwise AND &: a bit is set only if it is set in both operands

printf("%'06b", 0b110101 & 0b011001); // 010001

Bitwise OR |: a bit is set if it is set in either or both operands

printf("%'06b", 0b110101 | 0b011001); // 111101

Bitwise XOR ^: a bit is set if it is set in one operand and not set in another operand, i.e. only if that bit is in different state in the two operands

printf("%'06b", 0b110101 ^ 0b011001); // 101100

Example uses of bitmasks

These operators can be used to manipulate bitmasks. For example:

file_put_contents("file.log", LOCK_EX | FILE_APPEND);

Here, the | operator is used to combine the two bitmasks. Although \+ has the same effect, | emphasizes that you are combining bitmasks, not adding two normal scalar integers.

class Foo{
    const OPTION_A = 1;
    const OPTION_B = 2;
    const OPTION_C = 4;
    const OPTION_A = 8;

    private $options = self::OPTION_A | self::OPTION_C;

    public function toggleOption(int $option){
        $this->options ^= $option;
    }

    public function enable(int $option){
        $this->options |= $option; // enable $option regardless of its original state
    }

    public function disable(int $option){
        $this->options &= ~$option; // disable $option regardless of its original state,
                                    // without affecting other bits
    }

    /** returns whether at least one of the options is enabled */
    public function isOneEnabled(int $options) : bool{
        return $this->options & $option !== 0;
        // Use !== rather than >, because 
        // if $options is about a high bit, we may be handling a negative integer
    }

    /** returns whether all of the options are enabled */
    public function areAllEnabled(int $options) : bool{
        return ($this->options & $options) === $options;
        // note the parentheses; beware the operator precedence
    }
}

This example (assuming $option always only contain one bit) uses:

• the ^ operator to conveniently toggle bitmasks.
• the | operator to set a bit neglecting its original state or other bits
• the ~ operator to convert an integer with only one bit set into an integer with only one bit not set
• the & operator to unset a bit, using these properties of &:
• Since &= with a set bit will not do anything ((1 & 1) === 1, (0 & 1) === 0), doing &= with an integer with only one bit not set will only unset that bit, not affecting other bits.
• &= with an unset bit will unset that bit ((1 & 0) === 0, (0 & 0) === 0)
• Using the & operator with another bitmask will filter away all other bits not set in that bitmask.
• If the output has any bits set, it means that any one of the options are enabled.
• If the output has all bits of the bitmask set, it means that all of the options in the bitmask are enabled.

Bear in mind that these comparison operators: (\< \> <= >= == === != !== <> <=>) have higher precedence than these bitmask-bitmask operators: (| ^ &). As bitwise results are often compared using these comparison operators, this is a common pitfall to be aware of.

Bit-shifting operators

Bitwise left shift <<: shift all bits to the left (more significant) by the given number of steps and discard the bits exceeding the int size

<< $x is equivalent to unsetting the highest $x bits and multiplying by the $xth power of 2

printf("%'08b", 0b00001011<< 2); // 00101100

assert(PHP_INT_SIZE === 4); // a 32-bit system
printf("%x, %x", 0x5FFFFFFF << 2, 0x1FFFFFFF << 4); // 7FFFFFFC, FFFFFFFF

Bitwise right shift >>: discard the lowest shift and shift the remaining bits to the right (less significant)

>> $x is equivalent to dividing by the $xth power of 2 and discard the non-integer part

printf("%x", 0xFFFFFFFF >> 3); // 1FFFFFFF

Example uses of bit shifting:

Fast division by 16 (better performance than /= 16)

$x >>= 4;

On 32-bit systems, this discards all bits in the integer, setting the value to 0. On 64-bit systems, this unsets the most significant 32 bits and keep the least

$x = $x << 32 >> 32;

significant 32 bits, equivalent to $x & 0xFFFFFFFF

Note: In this example, printf("%'06b") is used. It outputs the value in 6 binary digits.