How to test PHP parameter input bitwise functions
Sometimes in programming, they allow you to bind parameters to one input variable of a function, just 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 symbol (|)
bitwise OR
. How do I add something inside foo()
to check $vFlags
to see what flags have been set?
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I think you will find that flags like this are usually defined as authority 2, for example:
define('FLAGA',1);
define('FLAGB',2);
define('FLAGC',4); /* then 8, 16, 32, etc... */
As you rightly said, they can be combined using the bitwise OR operator:
foo('test.txt',FLAGA | FLAGB | FLAGC);
To check these flags inside your function, you need to use the bitwise AND operator like this:
function foo($sFile, $vFlags) {
if ($vFlags & FLAGA) {
// FLAGA was set
}
if ($vFlags & FLAGB) {
// FLAGB was set
}
//// etc...
}
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The "flags" parameter will be referred to as the bitmask. One byte contains 8 bits that are either set or not set. You simply assign your meaning to each bit; if set, it means yes for that particular bit, otherwise no.
So, you need to start by defining your flags with the correct values that set the correct bits; just arbitrary numbers won't be concatenated into correct paths:
define('FLAGA', 1); // 00000001
define('FLAGB', 2); // 00000010
define('FLAGC', 4); // 00000100
define('FLAGD', 8); // 00001000
Considering the above, FLAGB | FLAGD
creates a bit mask with the second and fourth bits ( 00001010
). To do this, you need to slightly improve the conversion between base 2 (binary) and base 10 (decimal) for this.
To test this, you use &
:
$flags = FLAGB | FLAGD;
if ($flags & FLAGA) {
echo 'flag A is set';
}
if ($flags & FLAGB) {
echo 'flag B is set';
}
..
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You need a bitwise operator AND
&
:
define('FLAGA',40);
define('FLAGB',10);
define('FLAGC',3);
function foo($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);
However, bitwise operations necessarily work in binary terms, where each bit is a power of 2. Thus, you generally 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 of 40 for $vFlags
, you cannot determine which flags are set; it can be any of the following:
FLAGA & FLAGB
FLAGA & FLAGB & FLAGC
FLAGA & FLAGC
FLAGB & FLAGC
FLAGC
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