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Date: Fri, 16 May 2014 16:08:59 +0100
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To: internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Cannot use const within try block
From: daverandom@php.net (Chris Wright)

On 16 May 2014 15:44, Alain Williams <addw@phcomp.co.uk> wrote:
> The code below gives me: PHP Parse error:  syntax error, unexpected T_CONST in ...
>
> try {
>     const XXX = 12;
>
>     echo "XXX = " . XXX . "\n";
>
> }catch (ErrorException $e) {
>     echo "error {$e->getMessage()}\n";
> }
>
> It works if I put the 'cost' before/outside the 'try'.
>
> It also works if I replace the const line by:
>
>     define('XXX', 'twelve');
>
> or
>
>     include "testdef.php";
>
> where testdef.php contains:
>
>     const XXX = 12;
>

Defining constants using const is handled at compile time. Because of
this, it cannot be dependent on branches that are only processed at
run time. When you include a file, this creates a new "compile-time"
for the included file, so the operation succeeds.

define() is a regular function call evaluated at run time.

As such if you want a constant defined conditionally then you'll need
to use define()

> I cannot find this behaviour documented.
>
> a) If there is no good reason for this restriction - can it be removed.
>
> or
>
> b) If there is good reason for it can this be documented.

This behaviour is documented; to quote the note at http://php.net/const

"As opposed to defining constants using define(), constants defined
using the const keyword must be declared at the top-level scope
because they are defined at compile-time. This means that they cannot
be declared inside functions, loops or if statements."

I will update the note shortly to include try/catch blocks.

Thanks, Chris