PHP RFC: Constant Scalar Expressions

• Date: 2013-11-03
• Author: Bob Weinand bwoebi@php.net
• Status: Implemented in PHP 5.6
• First Published at: https://wiki.php.net/rfc/const_scalar_exprs
  • This is a follow-up to https://wiki.php.net/rfc/const_scalar_expressions from Anthony Ferrara ircmaxell@php.net (which was without constant support)

This RFC brings static scalar expressions to the parser. This allows places that only take static values (const declarations, property declarations, function arguments, etc) to also be able to take static expressions.

This can allow for writing far easier to understand code, by allowing for far more expressive code.

The main difference to Anthony's RFC is (apart from a few operators more) that constants can be involved in these scalar operations:

const_scalar_exprs_diff.php

<?php
 
const a = 1;
 
const b = a?2:100; // here the value of the constant "b" is dependent on the constant "a"
 
?>

Adding parser support for scalar expressions: operations on constants or constant values.

The following operations are currently supported by this proposal:

• + - Addition
• - - Subtraction
• * - Multiplication
• / - Division
• % - Modulus
• ! - Boolean Negation
• ~ - Bitwise Negation
• | - Bitwise OR
• & - Bitwise AND
• ^ - Bitwise XOR
• << - Bitwise Shift Left
• >> - Bitwise Shift Right
• . - Concatenation
• ?: - Ternary Operator
• <= - Smaller or Equal
• => - Greater or Equal
• == - Equal
• != - Not Equal
• < - Smaller
• > - Greater
• === - Identical
• !== - Not Identical
• && / and - Boolean AND
• || / or - Boolean OR
• xor - Boolean XOR

Also supported is grouping static operations: (1 + 2) * 3.

• 123 - Integers
• 123.456 - Floats
• “foo” - Strings
• __LINE__ - Line magic constant
• __FILE__ - File magic constant
• __DIR__ - Directory magic constant
• __TRAIT__ - Trait magic constant
• __METHOD__ - Method magic constant
• __FUNCTION__ - Function magic constant
• __NAMESPACE__ - Namespace magic constant
• <<<HEREDOC - HEREDOC string syntax (without variables)
• <<<'NOWDOC' - NOWDOC string syntax
• SOME_RANDOM_CONSTANT - Constants
• class_name::SOME_CONST - Class constants

constant_declarations.php

<?php
const FOO = 1 + 1;
const BAR = 1 << 1;
const GREETING = "HELLO";
const BAZ = GREETING." WORLD!"
?>

class_constant_declarations.php

<?php
class Foo {
    const FOO = 1 + 1;
    const BAR = 1 << 1;
    const GREETING = "HELLO";
    const BAZ = self::GREETING." WORLD!"
}
?>

class_property_declarations.php

<?php
class Foo {
    const BAZ = 10;
}
 
class Bar {
    public $foo = 1 + 1;
    public $bar = [
        1 + 1,
        1 << 2,
        Foo::BAZ => "foo "."bar"
    ];
    public $baseDir = __DIR__ . "/base";
}
?>

function_argument_declarations.php

<?php
const BAR = 1;
 
function foo($a = 1 + 1, $b = 2 << 3, $c = BAR?10:100) {
}
?>

static_variable_declarations.php

<?php
const BAR = 0x10;
 
function foo() {
    static $a = 1 + 1;
    static $b = [1 << 2];
    static $c = 0x01 | BAR;
}
?>

None

PHP 5.NEXT

An implementation based off of current master is available: Implementation On GitHub (Diff On GitHub)

The patch is ready to be merged. (Opcache support is included, thanks to Dmitry)

The implementation of the scalar expressions is based on an AST. That AST implementation eventually could be used later as a general-purpose AST for compiler with a few tweaks.

Should the patch for this RFC be merged into PHP 5.6?
Real name	Yes	No
ajf (ajf)
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dm (dm)
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treffynnon (treffynnon)
tyrael (tyrael)
willfitch (willfitch)
yohgaki (yohgaki)
Final result:	16	2
This poll has been closed.

The vote started the 20th November 2013 and ended the 27th November 2013.