如何在PHP中使用常量？

Question

I would like to use constants in PHP but "WEB" gets interpreted as false . I never used constants before, what do I miss?

define("WEB", true);
define("MOBILE", false);
define("DESKTOP", false);


if (defined('MOBILE' == true) || defined('DESKTOP' == true) ){
echo "MOBILE or DESKTOP";
} else if (defined('WEB' == true)) {
echo "WEB";
}

Answer 1

You are using defined() incorrectly. You are not checking if the constants are defined. You are checking their values. Just check them like you would a variable:

if (MOBILE == true || DESKTOP == true ){
echo "MOBILE or DESKTOP";
} else if (WEB == true) {
echo "WEB";
}

Which can be shortened to:

if (MOBILE || DESKTOP){
echo "MOBILE or DESKTOP";
} else if (WEB) {
echo "WEB";
}

Answer 2

defined gives you whether or not the constant exists, not its value.

if (MOBILE || DESKTOP){
  echo "MOBILE or DESKTOP";
} else if (WEB) {
  echo "WEB";
}

Answer 3

If you want to check if constant is defined, your code should be:

define("WEB", true);
define("MOBILE", false);
define("DESKTOP", false);

if(defined('MOBILE') || defined('DESKTOP'))
  echo "either MOBILE or DESKTOP is defined";
elseif(defined('WEB')) echo "WEB is defined";

If you want to check if constant value is true or false it would be

if(MOBILE || DESKTOP)
  echo "either MOBILE or DESKTOP is true";
elseif(WEB) echo "WEB is true";