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ZCE 5.3

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Bitwise operators



	// AND	: $a & $b (bits set in both $a and $b)
	// OR	: $a | $b (bits set in either $a or $b)
	// XOR	: $a ^ $b (bits set in $a or $b but not both)
	// NOT	: ~$a (negats bits - Convert 0s in 1s; 1s into 0s)
	// SHIFT : $a>>2 (move bits by 2 times)


Binary math is a BASE 2 numbering system, where as our normal everyday numbering system is a 
BASE 10 system

That's a normal everyday decimal number.

4768 = 4000 + 700 + 60 + 8 = 4 * 10 * 10 * 10 + 7 * 10 * 10 + 6 * 10 + 8
The BASE 2 system has the same concept however it goes by 2's instead of 10s
45 = 32 + 8 + 4 + 1 = // 00101101 0 * 2 * 2 * 2 * 2 * 2 * 2 * 2 + 0 * 2 * 2 * 2 * 2 * 2 * 2 + 1 * 2 * 2 * 2 * 2 * 2 + 0 * 2 * 2 * 2 * 2 + 1 * 2 * 2 * 2 + 1 * 2 * 2 + 0 * 2 + 1
A bit is a representation of 1 or 0. A byte is made up of 8 bits and the highest value of a byte is 255, which would mean every bit is set. 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128 = 255 AND (&) Bits that are set in both $a and $b are set
<?php $a = 9; // 8 + 1; 00001001 $b = 10; // 8 + 2; 00001010 echo $a & $b; // 8 $a = 36; // 32 + 4; 00100100 $b = 103; // 64 + 32 + 4 + 2 + 1; 01100111 echo $a & $b; // 36
  OR (|) Bits that are set in either $a or $b are set
<?php $a = 9; // 8 + 1; 00001001 $b = 10; // 8 + 2; 00001010 echo $a | $b; // 11
  XOR (^) Bits that are set in $a or $b but not both are set
<?php $a = 9; // 8 + 1; 00001001 $b = 10; // 8 + 2; 00001010 echo $a ^ $b; // 3
  NOT (~) The NOT operator wants to know what is set in $a but NOT set in $b
<?php $a = 9; // 8 + 1; 00001001 $b = 10; // 8 + 2; 00001010 echo $a & ~$b; // 1 $a = 9; // 8 + 1; 00001001 $b = 10; // 8 + 2; 00001010 echo ~$a & $b; // 2
  Shift (>>) Shift the bits of $a $b steps
<?php $a = 16; // 00010000 echo $a << 2; // 01000000 = 64 $a = 16; // 00010000 echo $a >> 2; // 00000100 = 4
  It's interesting to note that these last two operations provide an easy (and very fast) way ofmultiplying integers by a power of two.
<?php $a = 1; // 00000001 echo $a << 2; // 00000100 = 4 $a = 8; // 00001000 echo $a >> 2; // 00000010 = 2
  Be aware that there are overflow and underflow scenarios that can yield unexpected results. For example, on a 32-bit machine, the following will happen
<?php $a = 1; // 00000001 echo $a << 8; // 10000000 = 256 echo $a * pow(2, 8); // 256 $a = 1; // 00000001 echo $a << 64; // outputs 0, because all the bits have been shifted out of the integer value echo $a * pow(2, 64); // 4294967296
  Example of using bitwise operators (this is great if you have many classes of persmission for users)
<?php $read = 1; $write = 2; $readwrite = 16; $local_admin = 32; $global_admin = 64; $jim = 96; $mike = 16; if($mike & 32) {} // return 16 // Mike is allowed to edit; // being able to store all those in a single integer is VERY handy // instead of having to do something like if($is_local_admin && $is_global_admin && $can_edit_articles && $can_view_articles) {}
  Bitwise shift
<?php //echo str_pad('', 25, '1'); // 1111111111111111111111111 // 25 of 1 //echo str_pad(1, 25, 1); // 1111111111111111111111111 // 25 of 1 //echo bindec('1111') >> 2; // 3 (0011) // --- Shift function --- //echo str_pad(decbin(bindec('1111') >> 2), 4, '0', STR_PAD_LEFT); // 0011 function shift($str, $i) { return str_pad(decbin(bindec($str) >> $i), strlen($str), '0', STR_PAD_LEFT); } //echo shift('1111', 2); // 0011 //echo shift('111111', 3); // 000111 // --- Shift function (starting from char) --- function shift2($str, $i, $start=0) { if ($start > strlen($str)) return $str; if ($start < 0) return $str; $left = substr($str, 0, $start); $right = substr($str, $start, strlen($str)); return $left . str_pad(decbin(bindec($right) >> $i), strlen($right), '0', STR_PAD_LEFT); } //echo shift2('11111', 2); // 00111 //echo shift2('11111', 2, 1); // 10011 //echo shift2('11111', 2, 2); // 11001 //echo shift2('11111', 2, 3); // 11100 //echo shift2('11111', 2, 4); // 11110 //echo shift2('11111', 2, 7); // 11111
 


http://www.litfuel.net/tutorials/bitwise.htm