PHP RFC: Arbitrary Expression Interpolation

• Version: 0.9
• Date: 2017-09-10
• Author: Thomas Punt, tpunt@php.net
• Status: Draft
• First Published at: https://wiki.php.net/rfc/arbitrary_expression_interpolation

Interpolation is a widely used feature in PHP. It is, however, constrained to variable-like expressions only, which makes its usage somewhat limited. I would therefore like to propose for the ability to interpolate arbitrary expressions in PHP.

This proposal introduces a new interpolation syntax: #{}. Any expression within the parentheses will be evaluated and stringified, concatenating it to the rest of the string. This change affects double-quoted strings, heredocs, and the execution operator (shell execution via backticks).

This will:

• Give greater flexibility to developers when constructing strings from any expressions
• Make heredocs a more useful feature, since they will not be constrained to evaluating variable-like expressions only

Some examples:

$value = 10;
 
function someFunc()
{
    return "def";
}
 
var_dump(
    "Result: #{$value * 5}", // string(10) "Result: 50"
    "abc #{someFunc()}", // string(7) "abc def"
     `echo #{$value * 5}` // string(3) "50\n"
);
 
echo <<<END
    Result: #{$value * 5}
END; // "    Result: 50"

There were a few different candidates regarding the syntactic choice, including:

• ${} - Poses a very large BC break, since ${a} would now look for a constant (rather than a variable) named a
• {} - Poses a potentially large BC break by suddenly giving all curly braces in strings semantic meaning
• #{} - Poses a minor BC break, and is used by some other languages too (Ruby, Crystal, Elixir, CoffeeScript)

The new syntax will now cause the character sequence #{...} to be evaluated within strings.

The next PHP 7.x version (or 8.0, whichever comes next)

Still need to be checked.

A simple yes or no for this feature (with a 2/3 +1 majority required).

Initial implementation: https://github.com/php/php-src/compare/master...tpunt:arbitrary-expression-interpolation

Language specification: will be updated if the RFC is accepted.

After the project is implemented, this section should contain

1. the version(s) it was merged to
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature
4. a link to the language specification section (if any)

Links to external references, discussions or RFCs

Keep this updated with features that were discussed on the mail lists.