The constant name can be an empty string.
Code:
define("", "foo");
echo constant("");
Output:
foo
(PHP 4 >= 4.0.4, PHP 5, PHP 7)
constant — Returns the value of a constant
Return the value of the constant indicated by
name.
constant() is useful if you need to retrieve the value of a constant, but do not know its name. I.e. it is stored in a variable or returned by a function.
This function works also with class constants.
nameThe constant name.
Returns the value of the constant, or NULL if the constant is not
defined.
An E_WARNING level error is generated if the
constant is not defined.
Example #1 constant() example
<?php
define("MAXSIZE", 100);
echo MAXSIZE;
echo constant("MAXSIZE"); // same thing as the previous line
interface bar {
const test = 'foobar!';
}
class foo {
const test = 'foobar!';
}
$const = 'test';
var_dump(constant('bar::'. $const)); // string(7) "foobar!"
var_dump(constant('foo::'. $const)); // string(7) "foobar!"
?>
The constant name can be an empty string.
Code:
define("", "foo");
echo constant("");
Output:
foo
As of PHP 5.4.6 constant() pays no attention to any namespace aliases that might be defined in the file in which it's used. I.e. constant() always behaves as if it is called from the global namespace. This means that the following will not work:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant('F::BAR');
?>
However, calling constant('Foo::BAR') will work as expected.
Technically you can define constants with names that are not valid for variables:
<?php
// $3some is not a valid variable name
// This will not work
$3some = 'invalid';
// This works
define('3some', 'valid');
echo constant('3some');
?>
Of course this is not a good practice, but PHP has got you covered.
Maybe this is useful:
$file_ext is the file Extension of the image
<?php
if ( imagetypes() & @constant('IMG_' . strtoupper($file_ext)) )
{
$file_ext = $file_ext == 'jpg' ? 'jpeg' : $file_ext;
$create_func = 'ImageCreateFrom' . $file_ext;
}
?>
If you are referencing class constant (either using namespaces or not, because one day you may want to start using them), you'll have the least headaches when doing it like this:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant(F::class.'::BAR');
?>
since F::class will be dereferenced to whatever namespace shortcuts you are using (and those are way easier to refactor for IDE than just plain strings with hardcoded namespaces in string literals)
To access the value of a class constant use the following technique.
<?php
class a {
const b = 'c';
}
echo constant('a::b');
// output: c
?>
In reply to VGR_experts_exchange at edainworks dot com
To check if a constant is boolean, use this instead:
<?php
if (TRACE === true) {}
?>
Much quicker and cleaner than using defined() and constant() to check for a simple boolean.
IMO, using ($var === true) or ($var === false) instead of ($var) or (!$var) is the best way to check for booleans no matter what. Leaves no chance of ambiguity.
When you often write lines like
<?php
if(defined('FOO') && constant('FOO') === 'bar')
{
...
}
?>
to prevent errors, you can use the following function to get the value of a constant.
<?php
function getconst($const)
{
return (defined($const)) ? constant($const) : null;
}
?>
Finally you can check the value with
<?php
if(getconst('FOO') === 'bar')
{
...
}
?>
It's simply shorter.
This function is namespace sensitive when calling class constants.
Using:
<?php namespace sub;
class foo {
const BAR = 'Hello World';
}
constant('foo::BAR'); // Error
constant('sub\foo::BAR'); // works
?>
This does not seem to affect constants defined with the 'define' function. Those all end up defined in the root namespace unless another namespace is implicitly defined in the string name of the constant.
Checking if a constant is empty is bork...
You cannot
<?php
define('A', '');
define('B', 'B');
if (empty(B)) // syntax error
if (empty(constant('B'))) // fatal error
// so instead, thanks to LawnGnome on IRC, you can cast the constants to boolean (empty string is false)
if (((boolean) A) && ((boolean) B))
// do stuff
?>
Hello. This applies to constants being defined as Boolean values, and may-be applies generally.
I advise you to NOT use this in an included file, in a function or elsewhere outside the scope where the define('TRACE',TRUE) is placed) :
if (TRACE) {}
This will always evaluate to TRUE if the constant is not defined previously (the story about this becoming an string 'TRACE', thus evaluating to TRUE)
Use this :
<?php
if ((defined('TRACE'))AND(constant('TRACE'))) {}
?>
You can define values in your config file using the names of your defined constants, e.g.
in your php code:
define("MY_CONST",999);
in you config file:
my = MY_CONST
When reading the file do this:
$my = constant($value); // where $value is the string "MY_CONST"
now $my holds the value of 999
Return constants from an object. You can filter by regexp or match by value to find a constant name from the value.
Pretty useful sometimes.
<?php
function findConstantsFromObject($object, $filter = null, $find_value = null)
{
$reflect = new ReflectionClass($object);
$constants = $reflect->getConstants();
foreach ($constants as $name => $value)
{
if (!is_null($filter) && !preg_match($filter, $name))
{
unset($constants[$name]);
continue;
}
if (!is_null($find_value) && $value != $find_value)
{
unset($constants[$name]);
continue;
}
}
return $constants;
}
?>
Examples :
<?php
class Example
{
const GENDER_UNKNOW = 0;
const GENDER_FEMALE = 1;
const GENDER_MALE = 2;
const USER_OFFLINE = false;
const USER_ONLINE = true;
}
$all = findConstantsFromObject('Example');
$genders = findConstantsFromObject('Example', '/^GENDER_/');
$my_gender = 1;
$gender_name = findConstantsFromObject('Example', '/^GENDER_/', $my_gender);
if (isset($gender_name[0]))
{
$gender_name = str_replace('GENDER_', '', key($gender_name));
}
else
{
$gender_name = 'WTF!';
}
?>
It's easily to user constant() and define() to translate some words from your database-saves.
For example:
You have a table userprofil and one coloumn is "gender".
Gender can be male or female but you will display "maennlich" or "weiblich" (german words for it - whatever...)
First step: Fetch into $Gender
Second:
define("male", "maennlich");
define("female", "weiblich");
Third:
echo constant($Gender);
Now, the index of the variable $Gender will be handled like a constant!
(It works like "echo male;" for better understanding)
And a result of this, it displays maennlich btw. weiblich!
greetz
@XC:
That isn't necessary. If a constant is undefined, constant() returns NULL; simply suppressing the warning should be enough:
<?php
if(defined('FOO') && constant('FOO') === 'bar'){
// ...
}
?>
becomes
<?php
if(@constant('FOO') === 'bar') {
// ...
}
?>
Note that in the first snippet, the call to constant isn't unnecessary as well, and adds a bit of overhead. If you're set on using the first notation, the following is better:
<?php
if(defined('FOO') && FOO === 'bar') {
// ...
}
?>
howto echo CONSTANT_NAME without warnings and "if " checking:
<?php
!defined("CONSTANT_NAME") || constant("CONSTANT_NAME");
?>
This is how I check to see if a bool constant is true:
<?php
function consttrue($const) {
return !defined($const) ? false : constant($const);
}
?>
Examples
<?php
var_dump(consttrue('UNDEFINED_CONST'));
define('SOME_CONST', true);
var_dump(consttrue('SOME_CONST'));
define('SOME_CONST2', false);
var_dump(consttrue('SOME_CONST2'));
?>
Returns
bool(false)
bool(true)
bool(false)
If it isn't defined it will return false, otherwise it will return the value of the constant... which would be either true/false depending on what you set it to.