Accessing a class constant using a simple variable which contains the name of the constant

I'm trying to access a class constant in one of my classes:

const MY_CONST = "value";

If I have a variable which holds the name of this constant like this:

$myVar = "MY_CONST";

Can I access the value of MY_CONST somehow?

self::$myVar

does not work obviously because it is for static properties. Variable variables does not work either.

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have you tried 

$myVar = MY_CONST or $myVar = $MY_CONST
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Can I access the value of MY_CONST somehow?

self::MY_CONST

If you want to access is dynamically, you can use the reflection API Docs:

$myvar = 'MY_CONST';
$class = new ReflectionClass(self);
$const = $class->getConstant($myVar);

The benefit with the reflection API can be that you can get all constants at once (getConstants).

If you dislike the reflection API because you don't wanna use it, an alternative is the constant function (Demo):

$myvar = 'MY_CONST';
class foo {const MY_CONST = 'bar';}
define('self', 'foo');
echo constant(self.'::'.$myvar);
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There is no syntax for that, but you can use an explicit lookup:

print constant("classname::$myConst");

I believe it also works with self::.

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最佳答案

There are two ways to do this: using the constant function or using reflection.

Constant Function

The constant function works with constants declared through define as well as class constants:

class A
{
const MY_CONST = 'myval';


static function test()
{
$c = 'MY_CONST';
return constant('self::'. $c);
}
}


echo A::test(); // output: myval

Reflection Class

A second, more laborious way, would be through reflection:

$ref = new ReflectionClass('A');
$constName = 'MY_CONST';
echo $ref->getConstant($constName); // output: myval
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Just a note for Reflection: the constructor for ReflectionClass must receive the full path of the class for its parameter. This means that just setting the string 'A' as a constructor parameter may not work in some cases.

To avoid this problem, when using ReflectionClass you will be better if you do this:

$classA = new A();
$name_classA = get_class($classA);
$ref = new ReflectionClass(get_class($name_classA));
$constName = 'MY_CONST';
echo $ref->getConstant($constName);

Function get_class will give you the full path of a class whenever you are in the code. Missing the full path may result in a "Class not found" PHP error.


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