How To Test PHP Bitwise Function Input Parameters - php

Question

Sometimes in programming they allow one to chain parameters in a single function input variable like the second input variable below:
define('FLAGA',40);
define('FLAGB',10);
define('FLAGC',3);
function foo($sFile, $vFlags) {
// do something
}
foo('test.txt',FLAGA | FLAGB | FLAGC);
PHP calls this single pipe character (|) the Bitwise OR operator. How do I now add something inside foo() to test $vFlags to see which flags were set?

Answer 1

I think you'll find that flags like this are normally defined as powers of 2, e.g.:
define('FLAGA',1);
define('FLAGB',2);
define('FLAGC',4); /* then 8, 16, 32, etc... */
As you rightly stated, these can be combined by using a bitwise OR operator:
foo('test.txt',FLAGA | FLAGB | FLAGC);
To test these flags inside your function, you need to use a bitwise AND operator as follows:
function foo($sFile, $vFlags) {
if ($vFlags & FLAGA) {
// FLAGA was set
}
if ($vFlags & FLAGB) {
// FLAGB was set
}
//// etc...
}

Answer 2

The "flags" parameter would be called a bitmask. A single byte contains 8 bits which are either set or not set. You simply assign your own meaning to every bit; if it's set it means yes for that particular bit, otherwise no.
So you need to start by defining your flags with the correct values which set the right bits; just arbitrary numbers won't combine together in the right ways:
define('FLAGA', 1); // 00000001
define('FLAGB', 2); // 00000010
define('FLAGC', 4); // 00000100
define('FLAGD', 8); // 00001000
Given the above, FLAGB | FLAGD creates a bit mask with the second and forth bit set (00001010). You need to get somewhat comfortable with converting between base 2 (binary) and base 10 (decimal) for this.
To test for this, you use &:
$flags = FLAGB | FLAGD;
if ($flags & FLAGA) {
echo 'flag A is set';
}
if ($flags & FLAGB) {
echo 'flag B is set';
}
..

Answer 3

You need the bitwise AND operator &:
define('FLAGA',40);
define('FLAGB',10);
define('FLAGC',3);
function foo($sFile, $vFlags) {
if ($vFlags & FLAGA) {
echo "FLAGA is set\n";
}
if ($vFlags & FLAGB) {
echo "FLAGB is set\n";
}
if ($vFlags & FLAGC) {
echo "FLAGC is set\n";
}
}
foo('test.txt',FLAGA | FLAGB | FLAGC);
DEMO
That said, bitwise operations necessarily work in terms of binary, where each bit represents a power of 2. So, you are typically going to want to define flags in powers of 2, as in
define('FLAGA',1);
define('FLAGB',2);
define('FLAGC',4);
define('FLAGD',8); // etc.
Otherwise, imagine this scenario:
define('FLAGA',8);
define('FLAGB',32);
define('FLAGC',40);
If you have a value 40 for $vFlags, you can't tell what flags are set; it could be any of the following:
FLAGA & FLAGB
FLAGA & FLAGB & FLAGC
FLAGA & FLAGC
FLAGB & FLAGC
FLAGC