Dynamic constant name in PHP

Question

I am trying to create a constant name dynamically and then get at the value.

define( CONSTANT_1 , "Some value" ) ;

// try to use it dynamically ...
$constant_number = 1 ;
$constant_name = ("CONSTANT_" . $constant_number) ;

// try to assign the constant value to a variable...
$constant_value = $constant_name;

But I find that $constant value still contains the NAME of the constant, and not the VALUE.

I tried the second level of indirection as well $$constant_name But that would make it a variable not a constant.

Can somebody throw some light on this?

score 165 · Accepted Answer · answered Oct 22 '10 at 08:46

165

http://dk.php.net/manual/en/function.constant.php

echo constant($constant_name);

answered Oct 22 '10 at 08:46

Mads Lee Jensen

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score 88 · Answer 2 · answered Feb 27 '13 at 20:10

88

And to demonstrate that this works with class constants too:

class Joshua {
    const SAY_HELLO = "Hello, World";
}

$command = "HELLO";
echo constant("Joshua::SAY_$command");

answered Feb 27 '13 at 20:10

uɥƃnɐʌuop

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Worth noting that you may need to specify the fully qualified (namespaced) class name if the constant is in a class which is not in the current namespace - regardless of if you have added a "use" for the class in your file. – lopsided Aug 25 '14 at 12:32
1

This answer is great because of good example. That's exactly what I was looking for :) Thanks! – ElChupacabra Dec 31 '15 at 16:53
7

@lopsided The `::class` constant can be used to retrieve the fully qualified namespace, for example: `constant(YourClass::class . '::CONSTANT_' . $yourVariable);` – Willem-Aart Aug 27 '17 at 17:42
1

Note that the [`::class` keyword](http://php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class) is available since php 5.5 – T30 Jan 11 '18 at 10:30

score 9 · Answer 3 · answered Feb 18 '15 at 11:38

To use dynamic constant names in your class you can use reflection feature (since php5):

$thisClass = new ReflectionClass(__CLASS__);
$thisClass->getConstant($constName);

For example: if you want to filter only specific (SORT_*) constants in the class

class MyClass 
{
    const SORT_RELEVANCE = 1;
    const SORT_STARTDATE = 2;

    const DISTANCE_DEFAULT = 20;

    public static function getAvailableSortDirections()
    {
        $thisClass = new ReflectionClass(__CLASS__);
        $classConstants = array_keys($thisClass->getConstants());

        $sortDirections = [];
        foreach ($classConstants as $constName) {
            if (0 === strpos($constName, 'SORT_')) {
                $sortDirections[] =  $thisClass->getConstant($constName);
            }
        }

        return $sortDirections;
    }
}

var_dump(MyClass::getAvailableSortDirections());

result:

array (size=2)
  0 => int 1
  1 => int 2

Dynamic constant name in PHP

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