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To: Mike Schinkel <mike@newclarity.net>, PHP internals
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Thread-Topic: [PHP-DEV] Initializing constants once, with code?
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Date: Fri, 3 Jan 2020 00:09:31 +0000
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Subject: Re: [PHP-DEV] Initializing constants once, with code?
From: tysonandre775@hotmail.com (tyson andre)

> The problem with using constants is that the value is still hardcoded=0A=
> and if we later want to change to pulling the data from a config file =0A=
> we have to change all the code that uses those constants.=A0=0A=
=0A=
It's possible to work around this by using a unique constant name=0A=
and define('unique_name', $dynamicExpression)=0A=
(and const MY_CLASS_CONST =3D \unique_name;)=0A=
=0A=
Maybe php could add something along these lines:=0A=
=0A=
local const X =3D sprintf('In %s', $dynamicValue);=0A=
// Not supported in functions or loops.=0A=
// Translates internally to:=0A=
// if (defined('some_globally_unique_constant123af') || =0A=
//     !define('some_globally_unique_constant123af', X)) { throw ...; }=0A=
// use some_globally_unique_constant123af as X;=0A=
class MyClass {=0A=
    const PUBLIC =3D X;=0A=
}=0A=
=0A=
- php still needs to prevent recursion such as =0A=
  `const A =3D count(B); const B =3D array_fill(1, A, 'value');`,=0A=
  which is why the simplest approach would be one that eagerly evaluates dy=
namic expressions.=0A=
  That's probably not the only way - maybe it could track all of the consta=
nts that=0A=
  are in the process of being defined and cause an uncatchable fatal error =
if there was recursion.=0A=
- Right now, reading the value of a class constant can already emit notices=
 or throw an Error (undeclared constants)=0A=
  the first time it gets evaluated=0A=
- Any approach should throw for results that contain cycles, objects, or re=
ferences,=0A=
  the same way define() warns.=0A=
=0A=
The below ugly proof of concept shows that the php runtime can in a sense=
=0A=
defer evaluation of dynamic class constants to runtime (not recursively)=0A=
(assume errstr/errfile control what constant gets loaded).=0A=
This POC should not be used anywhere.=0A=
=0A=
<?php=0A=
set_error_handler(function ($errno, $errstr) {=0A=
    echo "Ignoring $errno: $errstr\n";=0A=
    define(=0A=
        'SOME_GLOBALLY_UNIQUE_CONSTANT',=0A=
        sprintf('Hello, %s', $GLOBALS['globalVar'])=0A=
    );=0A=
});=0A=
class MyTest {=0A=
    const KEY =3D 'myKey';=0A=
    const FOO =3D [=0A=
        self::KEY =3D> [[][0], SOME_GLOBALLY_UNIQUE_CONSTANT][1],=0A=
    ];=0A=
}=0A=
$globalVar =3D 'world';=0A=
echo "Fetching MyTest::FOO\n";=0A=
var_dump(MyTest::FOO);=0A=
/*=0A=
Output:=0A=
Fetching MyTest::FOO=0A=
Ignoring 8: Undefined offset: 0=0A=
array(1) {=0A=
  ["myKey"]=3D>=0A=
  string(12) "Hello, world"=0A=
}=0A=
 */=0A=