Question

I am trying to optimize my mysql table a little bit in order to have a slightly more manageable table. I would like to store user permissions in a bit field.

For example, a users permissions could be 0110 (I have a growing number of user permissions, so the length of this could be a bit longer)

The example might correspond to the following:

0: User cannot post news items on the website
1: User can post new articles on the website
1: User can edit any article on the website
0: User cannot remove articles from the website
etc. (for other permissions)

If I have this stored in a mysql bit field. How can I manipulate this in PHP?

For example, in PHP is there an easy way to get the 3rd bit and see if it is a 0 or a 1?

Is there an easy way to set the 3rd bit to be a 0 or a 1?

Answer 1

You can use bitwise operators.

For instance :

$bits = bindec('0110');
var_dump($bits & bindec('0100'));

Will get you “4” (i.e. non-zero — which you can consider as “ok”), as the third bit in $bits is set.

And :

$bits = bindec('0010');
var_dump($bits & bindec('0100'));

Will get you 0 — as the third bit in $bits is not set.

Of course, no need to call bindec each time — I just used it here to make things easier to read ; my first example could be rewritten as something like this :

$bits = bindec('0110');
var_dump($bits & 1<<2);

And to make to code easier to understand, don’t use such magic values, but use some constants ; for instance :

define('PERMISSION_TO_DO_X', 1);
define('PERMISSION_TO_DO_Y', 1<<1);
define('PERMISSION_TO_DO_Z', 1<<2);

And to use them :

$bits = bindec('0110');
if ($bits & PERMISSION_TO_DO_X) {
    echo "OK to do X<br />";
}
if ($bits & PERMISSION_TO_DO_Y) {
    echo "OK to do Y<br />";
}
if ($bits & PERMISSION_TO_DO_Z) {
    echo "OK to do Z<br />";
}

Which, here, gets you that kind of output :

OK to do Y
OK to do Z

Because :

• the right-most bit is 0 (permission X not granted)
• the second bit (starting from the right) is 1 : permission Y is granted
• the third bit (still and always starting from the right) is 1 : permission Z granted