The constant name can be an empty string.
Code:
define("", "foo");
echo constant("");
Output:
foo
(PHP 4 >= 4.0.4, PHP 5, PHP 7, PHP 8)
constant — 返回一个常量的值
返回 name 对应的常量的值。
当你不知道常量名，却需要获取常量的值时，constant() 就很有用了。也就是说，常量名储存在一个变量里，或者由函数返回时。
该函数也适用类常量和 enum cases。
name常量名。
返回常量的值。
如果常量未定义，会抛出 Error 异常。
在 PHP 8.0.0 之前，会产生 E_WARNING 级别的错误。
| 版本 | 说明 |
|---|---|
| 8.0.0 |
如果常量未定义，constant() 现在会抛出
Error 异常。以前会产生一个
E_WARNING 级别的错误并返回 null。
|
示例 #1 constant() 与常量一起使用
<?php
define("MAXSIZE", 100);
echo MAXSIZE;
echo constant("MAXSIZE"); // 和上行一样
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!"
?>示例 #2 constant() 和 Enum Cases 一起使用（自 PHP 8.1.0 起）
<?php
enum Suit
{
case Hearts;
case Diamonds;
case Clubs;
case Spades;
}
$case = 'Hearts';
var_dump(constant('Suit::'. $case)); // enum(Suit::Hearts)
?>
The constant name can be an empty string.
Code:
define("", "foo");
echo constant("");
Output:
fooIf you are referencing class constant (either using namespaces or not, because one day you may want to start using them), you'll have the least headaches when doing it like this:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant(F::class.'::BAR');
?>
since F::class will be dereferenced to whatever namespace shortcuts you are using (and those are way easier to refactor for IDE than just plain strings with hardcoded namespaces in string literals)As of PHP 5.4.6 constant() pays no attention to any namespace aliases that might be defined in the file in which it's used. I.e. constant() always behaves as if it is called from the global namespace. This means that the following will not work:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant('F::BAR');
?>
However, calling constant('Foo::BAR') will work as expected.It is worth noting, that keyword 'self' can be used for constant retrieval from within the class it is defined
<?php
class Foo {
const PARAM_BAR = 'baz';
public function getConst($name) {
return constant("self::{$name}");
}
}
$foo = new Foo();
echo $foo->getConst('PARAM_BAR'); // prints 'baz'
?>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' . $file_ext;
}
?>When you often write lines like
<?php
if(defined('FOO') && constant('FOO') === 'bar')
{
...
}
?>
to prevent errors, you can use the following function to get the value of a constant.
<?php
function getconst($const)
{
return (defined($const)) ? constant($const) : null;
}
?>
Finally you can check the value with
<?php
if(getconst('FOO') === 'bar')
{
...
}
?>
It's simply shorter.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.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
?>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.<?php
namespace Foo;
define(__NAMESPACE__ . '\Bar', 'its work'); // ..but IDE may make notice
echo Bar; // its workYou 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 $my holds the value of 999Return constants from an object. You can filter by regexp or match by value to find a constant name from the value.
Pretty useful sometimes.
<?php
function findConstantsFromObject($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!';
}
?>The use of constant() (or some other method) to ensure the your_constant was defined is particularly important when it is to be defined as either `true` or `false`.
For example, taken from this Stackoverflow Question
https://stackoverflow.com/questions/5427886/php-undefined-constant-testing/56604602#56604602)
If `BOO` did NOT get defined as a constant, for some reason,
<?php if(BOO) do_something(); ?>
would evaluate to `TRUE` and run anyway. A rather unexpected result.
The reason is that PHP ASSUMES you "forgot" quotation marks around `BOO` when it did not see it in its list of defined constants.
So it evaluates: `if ('BOO')`...
Since every string, other than the empty string, is "truthy", the expression evaluates to `true` and the do_something() is run, unexpectedly.
If you instead use:
<?php if (constant(BOO)) do_something() ?>
then if `BOO` has not been defined, `constant(BOO)` evaluates to `null`,
which is falsey, and `if (null)`... becomes `false`, so do_something() is skipped, as expected.
The PHP behavior regarding undefined constants is particularly glaring when having a particular constant defined is the exception, "falsey" is the default, and having a "truthy" value exposes a security issue. For example,
<?php if (IS_SPECIAL_CASE) show_php_info() ?> .
There are other ways around this PHP behavior, such as
<?php if (BOO === true) do_something(); ?>
or
<?php if (defined('BOO') && BOO) do_something() ?>.
Note that only the version using `defined()` works without also throwing a PHP Warning "error message."
Here is a php repl.it demonstration:
https://repl.it/@sherylhohman/php-undefined-constants-beware-of-truthy-conversion?language=php_cli&folderId=
(disclosure: I also submitted an answer to the SO question linked to above)// 1) you can store the name of constant in default variable
// and use it without identify it's name :)
$str= "constName";
define("constName","this is constant");
echo constant($str);
output:
this is constant
// 2) good for dynamic generating constants
function generateConst( $const , $value , $sensitivity=TRUE )
{
define( "$const" , "$value ",$sensitivity);
}
$CONST="cost";
$VALUE="100$";
generateConst( $CONST , $VALUE);
echo constant($const);
output:
100$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