Get value of dynamically chosen class constant in PHP

I would like to be able to do something like this:

class ThingIDs
{
    const Something = 1;
    const AnotherThing = 2;
}

$thing = 'Something';
$id = ThingIDs::$thing;

This doesn't work. Is there a straightforward way of doing something equivalent? Note that I'm stuck with the class; it's in a library I can't rewrite. I'm writing code that takes arguments on the command line, and I would really like it to take symbolic names instead of id numbers.

Use the constant() function:

$id = constant("ThingIDs::$thing");

Use Reflection

$r = new ReflectionClass('ThingIDs');
$id = $r->getConstant($thing);

If you are using namespaces, you should include the namespace with the class.

echo constant('My\Application\ThingClass::ThingConstant'); 

Helper function

You can use a function like this:

function class_constant($class, $constant)
{
    if ( ! is_string($class)) {
        $class = get_class($class);
    }

    return constant($class . '::' . $constant);
}

It takes two arguments:

• Class name or object instance
• Class constant name

If an object instance is passed, its class name is inferred. If you use PHP 7, you can use ::class to pass appropriate class name without having to think about namespaces.

Examples

class MyClass
{
    const MY_CONSTANT = 'value';
}

class_constant('MyClass', 'MY_CONSTANT'); # 'value'
class_constant(MyClass::class, 'MY_CONSTANT'); # 'value' (PHP 7 only)

$myInstance = new MyClass;
class_constant($myInstance, 'MY_CONSTANT'); # 'value'

<?php

class Dude {
    const TEST = 'howdy';
}

function symbol_to_value($symbol, $class){
    $refl = new ReflectionClass($class);
    $enum = $refl->getConstants();
    return isset($enum[$symbol])?$enum[$symbol]:false;
}

// print 'howdy'
echo symbol_to_value('TEST', 'Dude');

If you have a reference to the class itself then you can do the following:

if (defined(get_class($course). '::COURSES_PER_INSTANCE')) {
   // class constant is defined
}

My problem was similiar to this subject. When you have the object, but not the class name, you could use:

$class_name = get_class($class_object);
$class_const = 'My_Constant';
			
$constant_value = constant($class_name.'::'.$class_const);

I know I'm a bit late, but I hope this can help anyway.

Based on Phil's answer, I created a default enumerator class that can be extended.

class DefaultEnum
{
    static public function getConstantText(string $constant)
    {
        try {
            // Get child class name that called this method
            $child_class = get_called_class();

            $reflection = new ReflectionClass($child_class);
            $const = $reflection->getConstant($constant);

            return $const;
        } catch (\ReflectionException $e) {
            // ...
        }
    }
}

class CustomEnum extends DefaultEnum
{
    const something = 'abcd';
    const something2 = 'ABCD';
}

You can call this method like this

CustomEnum::getConstantText('something');

It will return 'abcd'.

The function get_called_class() is a function that returns the class name that called this method and it works specifically for static methods.

In this case $child_class value will be CustomEnum::class. ReflectionClass accepts strings and object as parameter.