CODEWITHSUNDEEP
phpfunctionbit-manipulationflags
Teiv
Teiv

Reputation: 2635

PHP function flags, how?

I'm attempting to create a function with flags as its arguments but the output is always different with what's expected :

define("FLAG_A", 1);  
define("FLAG_B", 4);  
define("FLAG_C", 7);  
function test_flags($flags) {  
 if($flags & FLAG_A) echo "A";  
 if($flags & FLAG_B) echo "B";  
 if($flags & FLAG_C) echo "C";   
}  
test_flags(FLAG_B | FLAG_C); # Output is always ABC, not BC

How can I fix this problem?

Upvotes: 22

Views: 16730

Answers (1)

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798814

Flags must be powers of 2 in order to bitwise-or together properly.

define("FLAG_A", 0x1);
define("FLAG_B", 0x2);
define("FLAG_C", 0x4);
function test_flags($flags) {
  if ($flags & FLAG_A) echo "A";
  if ($flags & FLAG_B) echo "B";
  if ($flags & FLAG_C) echo "C";
}
test_flags(FLAG_B | FLAG_C); # Now the output will be BC

Using hexadecimal notation for the constant values makes no difference to the behavior of the program, but is one idiomatic way of emphasizing to programmers that the values compose a bit field. Another would be to use shifts: 1<<0, 1<<1, 1<<2, &c.

Upvotes: 38

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