Best workaround to create a PHP class constant from an expression? - php

Question

I'd like to be able to do something like this:
class Circle {
const RADIUS_TO_CIRCUMFERENCE = M_PI * 2; // Not allowed
private $radius;
public function __construct( $radius ) {
$this->radius = $radius;
}
...
public function getCircumference() {
return $this->radius * self::RADIUS_TO_CIRCUMFERENCE;
}
}
But I can't create a class constant from an expression like that:
The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call.
So my question is: What's the best workaround for this limitation of PHP? I'm aware of the following workarounds, but are there any others which are better?
1. Create a property
class Circle {
private static $RADIUS_TO_CIRCUMFERENCE;
private $radius;
public function __construct( $radius ) {
$this->radius = $radius;
$this->RADIUS_TO_CIRCUMFERENCE = M_PI * 2;
}
...
public function getCircumference() {
return $this->radius * $this->RADIUS_TO_CIRCUMFERENCE;
}
}
I don't like this, because the value of $RADIUS_TO_CIRCUMFERENCE can be changed, so it's not really a "constant".
2. Use define()
define( 'RAD_TO_CIRCUM', M_PI * 2 );
class Circle {
const RADIUS_TO_CIRCUMFERENCE = RAD_TO_CIRCUM;
...
public function getCircumference() {
return $this->radius * self::RADIUS_TO_CIRCUMFERENCE;
}
}
This is better, since the value is truly constant, but the drawback is that RAD_TO_CIRCUM has been globally defined.
A digression
I don't understand how this can work. (Edit: I've tested it, and it does work.) According to the Handbook of PHP Syntax:
The const modifier creates a compile-time constant and so the compiler will replace all usage of the constant with its value. In contrast, define creates a run-time constant which is not set until run-time. This is the reason why define constants may be assigned with expressional values, whereas const requires constant values which are known at compile-time.
The manual confirms that "constants defined using the const keyword ... are defined at compile-time".
In this bug report from 3 years ago, a member of the PHP team wrote:
For the class constant we need a constant value at compile time and can't evaluate expressions. define() is a regular function, evaluated at run time and can therefore contain any value of any form.
But in my example above, the value of RAD_TO_CIRCUM is not known at compile-time. So what is the compiler putting for the value of RADIUS_TO_CIRCUMFERENCE?
I'm guessing that the compiler creates some kind of placeholder for the value of RADIUS_TO_CIRCUMFERENCE, and at run-time, that placeholder gets replaced with the value of RAD_TO_CIRCUM. Might this placeholder be a kind of resource? If so, maybe this technique should be avoided? The manual says: "It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results."
3. Create a method
class Circle {
...
private static function RADIUS_TO_CIRCUMFERENCE() {
return M_PI * 2;
}
public function getCircumference() {
return $this->radius * $this->RADIUS_TO_CIRCUMFERENCE();
}
}
This is my favourite workaround that I'm aware of. The value is constant, and it doesn't affect the global space.
Is there another workaround which is even better?

Answer 1

As of PHP 5.6, you can use math expressions in PHP constants, so this would work:
const RADIUS_TO_CIRCUMFERENCE = M_PI * 2;
I encountered this thread because my environment wasn't configured properly (was set to PHP 5.4 by accident), so don't forget to check your PHP version.

Answer 2

I would recommend this approach:
class Circle {
...
private function RADIUS_TO_CIRCUMFERENCE() {
static $RADIUS_TO_CIRCUMFERENCE;
if ( null === $RADIUS_TO_CIRCUMFERENCE )
$RADIUS_TO_CIRCUMFERENCE = M_PI * 2;
return $RADIUS_TO_CIRCUMFERENCE;
}
public function getCircumference() {
return $this->radius * $this->RADIUS_TO_CIRCUMFERENCE();
}
}
The goal is calculation only once for all class entities, like a real constant.

Answer 3

In my case,
my installed php version is 7.1 but, in my IDE(PhpStorm) settings my php version 5.4 was selected. Once i've changed my php version the problem was gone.
PhpStorm settings->PHP and change the PHP language level and CLI interpreter php version.

Answer 4

If you agree to use a standard variable instead of const keyword :
class Foo {
public $CONST_A; // = calculation A // To let people quickly see the value.
public $CONST_B; // = calculation B
public static function initClass() {
self::$CONST_A = /* calculation A */;
self::$CONST_B = /* calculation B */;
}
}
Foo::initClass();
initClass() is done only once, when the class file is required.