PHP and MySQL®: Create-Modify-Reuse

2.3. Working with Permissions and Bitwise Operators

A basic understanding of bit operators and how computers store information is all it takes to understand how our permission scheme will work. The operators I'll focus on are the bitwise-and (&) and bitwise-or (|) operators.

Think of a standard light switch; the circuit can have two states. It's either closed with electricity flowing through it or open so no charge can flow. Each byte that makes up a number stored in a computer is ultimately nothing more than a series of tiny electrical switches, with on (closed) and off (open) values. The computer interprets a series of the consecutive on/off values into something interesting for us humans, such as a letter or a number.

Bitwise operators such as bitwise-and and bitwise-or can compare specific bits in the sequences and flip them on or off. If a sequence of switches or bits changes, then the meaning does as well. It's customary to represent the on state with the number 1 and off with 0. Take a brief look at how the following numbers 0 through 24 are represented:

0 0000 0000	1 0000 0001	2 0000 0010	3 0000 0011	4 0000 0100
5 0000 0101	6 0000 0110	7 0000 0111	8 0000 1000	9 0000 1001
10 0000 1010	11 0000 1011	12 0000 1100	13 0000 1101	14 0000 1110
14 0000 1111	16 0001 0000	17 0001 0001	18 0001 0010	19 0001 0011
20 0001 0100	21 0001 0101	22 0001 0110	23 0001 0111	24 0001 1000

Notice there is something special about the numbers 1, 2, 4, 8 and 16. In each of these values the left-most 1-bit advances a placeholder. ...