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(PHP 4 >= 4.0.4, PHP 5, PHP 7)
constant — 返回一个常量的值
通过 name 返回常量的值。
当你不知道常量名，却需要获取常量的值时，constant() 就很有用了。也就是常量名储存在一个变量里，或者由函数返回常量名。
该函数也适用 class constants。
name常量名。
返回常量的值。如果常量未定义则返回 NULL。
如果常量未定义，会产生一个 E_WARNING 级别的错误。
Example #1 constant() 的例子
<?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!"
?>
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// 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 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 $foo->getConst('PARAM_BAR'); // prints 'baz'
?>
If you are referencing class constant (either using namespaces or not, because one day you may want to start using them), you'll have the least headaches when doing it like this:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant(F::class.'::BAR');
?>
since F::class will be dereferenced to whatever namespace shortcuts you are using (and those are way easier to refactor for IDE than just plain strings with hardcoded namespaces in string literals)
As of PHP 5.4.6 constant() pays no attention to any namespace aliases that might be defined in the file in which it's used. I.e. constant() always behaves as if it is called from the global namespace. This means that the following will not work:
<?php
class Foo {
const BAR = 42;
}
?>
<?php
namespace Baz;
use \Foo as F;
echo constant('F::BAR');
?>
However, calling constant('Foo::BAR') will work as expected.
This function is namespace sensitive when calling class constants.
Using:
<?php namespace sub;
class foo {
const BAR = 'Hello World';
}
constant('foo::BAR'); // Error
constant('sub\foo::BAR'); // works
?>
This does not seem to affect constants defined with the 'define' function. Those all end up defined in the root namespace unless another namespace is implicitly defined in the string name of the constant.
Return constants from an object. You can filter by regexp or match by value to find a constant name from the value.
Pretty useful sometimes.
<?php
function findConstantsFromObject($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!';
}
?>
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
?>
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 $my holds the value of 999
When you often write lines like
<?php
if(defined('FOO') && constant('FOO') === 'bar')
{
...
}
?>
to prevent errors, you can use the following function to get the value of a constant.
<?php
function getconst($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
?>
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
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
$3some = 'invalid';
// This works
define('3some', 'valid');
echo constant('3some');
?>
Of course this is not a good practice, but PHP has got you covered.
In reply to VGR_experts_exchange at edainworks dot com
To check if a constant is boolean, use this instead:
<?php
if (TRACE === true) {}
?>
Much quicker and cleaner than using defined() and constant() to check for a simple boolean.
IMO, using ($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.
The constant name can be an empty string.
Code:
define("", "foo");
echo constant("");
Output:
foo
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;
}
?>