PHP Checking if constant is defined


Example

Simple check

To check if constant is defined use the defined function. Note that this function doesn't care about constant's value, it only cares if the constant exists or not. Even if the value of the constant is null or false the function will still return true.

<?php

define("GOOD", false);

if (defined("GOOD")) {
    print "GOOD is defined" ; // prints "GOOD is defined"

    if (GOOD) {
        print "GOOD is true" ; // does not print anything, since GOOD is false
    }
}

if (!defined("AWESOME")) {
   define("AWESOME", true); // awesome was not defined. Now we have defined it 
}

Note that constant becomes "visible" in your code only after the line where you have defined it:

<?php

if (defined("GOOD")) {
   print "GOOD is defined"; // doesn't print anyhting, GOOD is not defined yet.
}

define("GOOD", false);

if (defined("GOOD")) {
   print "GOOD is defined"; // prints "GOOD is defined"
}

Getting all defined constants

To get all defined constants including those created by PHP use the get_defined_constants function:

<?php

$constants = get_defined_constants();
var_dump($constants); // pretty large list

To get only those constants that were defined by your app call the function at the beginning and at the end of your script (normally after the bootstrap process):

<?php

$constants = get_defined_constants();

define("HELLO", "hello"); 
define("WORLD", "world"); 

$new_constants = get_defined_constants();

$myconstants = array_diff_assoc($new_constants, $constants);
var_export($myconstants); 
   
/* 
Output:

array (
  'HELLO' => 'hello',
  'WORLD' => 'world',
) 
*/

It's sometimes useful for debugging