Editing PHP Skills Tests (section)

== Bitwise operators ==

{| class="wikitable"
! Example
! Name
! Result
|-
| `$a & $b`
| And 
| Bits that are set in both `$a` and `$b` are set.
|-
| `<nowiki>$a | $b</nowiki>`
| Or (inclusive or)
| Bits that are set in either `$a` or `$b` are set.
|- 
| `$a ^ $b`
| Xor (exclusive or)
| Bits that are set in `$a` or `$b` but not both are set.
|- 
| `~ $a`
| Not 
| Bits that are set in `$a` are not set, and vice versa.
|-
| `$a << $b`
| Shift left
| Shift the bits of `$a` `$b` steps to the left (each step means "multiply by two")
|-
| `$a >> $b`
| Shift right
| Shift the bits of `$a` `$b` bits to the right (each step means "divide by two")
|}

=== Bitwise operators ===

<syntaxhighlight lang="php">
$a = (1<<0); /* $a = (0001) = (1*1) = 1 */
$b = (1<<1); /* $b = (0010) = (1*2) = 2 */

echo $b | $a; /* (0010 | 0001) = (0011) = ((1*2) + (1*1)) = 3 (bits set in either value are set in the result) */
</syntaxhighlight>

=== Bitwise operators and precedence ===

'''Q: What will be the output of the following code?'''

<syntaxhighlight lang="php">
$a = (1 << 0); /* $a = (1*1) = 1 */
$b = (1 << $a); /* $b = (1*2) = 2 */
$c = (1 << $b); /* $c = (1*4) = 4 */
echo ($c || $b) << 2 * $a | $a; 
/* ($c || $b) i.e. "4 or 2". Unless they are both false it will evaluated to true. Result is "true", or 1 */
/* multiplication > bitwise shift > bitwise or, in terms of precedence */
/* next operation, multiplication: 2 * $a, or 2 * 1 = 2 */
/* next operation, bitwise shift: 1 << 2 = 4; */
/* next operation, bitwise or: 4 | 1 = 100 | 001 = 5 */ 
</syntaxhighlight>

'''A: 5'''

=== Bit shifting ===

'''Q: What will be the output of the following code?'''

<syntaxhighlight lang="php">
$a = 0x01; // 1
$b = 0x02; // 2

echo $a === $b >> $a; 
</syntaxhighlight>

{|
| '''a.'''
| 0
|-
| '''b.'''
| 1
|-
| '''c.'''
| Syntax error
|}

'''A: (b) 1'''

* `$a` equals 1
* `$b >> $a` shifts the bit in 2 over one place, resulting in 1
* `$a === $b >> $a` translates to "does 1 equal 1?"
* Answer is "yes", i.e. true, i.e. 1

----

'''Q: What will be the result of the following code?'''

<syntaxhighlight lang="php">
$a = 0x01; // 1
$b = 0x02; // 2

printf("%x", ($a << $b.$b)); // 400000
</syntaxhighlight>

* `%x` format for `printf()` means output a hexidecimal value using lowercase letters.
* `$b.$b` evaluates to `22`.
* So, shift a bit representing 1 over 22 places, i.e. 2 to the power of 22.
* When you shift the bit over, you multiply the value by 2, so the progression is like 1, 2, 4, 8, and then `8 * 2 = 16` which in hex is 10. Then `10 * 2` in hex results in 20 which is really `16 * 2` or `2**5` or `32`.
* An algorithm for translating the shift to hex would be `(2**($shift_value % 4)) . str_pad(<nowiki>''</nowiki>, floor($shift_value/4), '0')`

----

'''Q: What will be the output of the following code?'''

<syntaxhighlight lang="php">
$a = (1<<0); // 0b0001, or 1
$b = (1<<1); // 0b0010, or 2 

echo ($b|$a) << 2;
</syntaxhighlight>

{|
! a.
| 1
|-
! b.
| 2
|-
! c.
| 3
|-
! d.
| 4
|-
! e.
| 8
|-
! f.
| 12
|-
! g.
| Syntax error
|}

'''A: (f) 12'''

* `($b|$a)` Or operator set the bit if it's set in either `$b` or `$a`, or `0b0010 | 0b0001` = `0b0011`, or `3`
* Shift 3 two places yields `0b1100` or `12`
* Also, shifting a bit left in effect multiplies the value by 2 for each time the bit is shifted, so `3 * 2 * 2 = 12`
* Also, shifting a bit left if effect multiplies the value by 2 to the power of the shift amount, e.g. `3 * (2**2) = 3 * 4 = 12`