The PHP team has released PHP 8.3 today with typed class constants, a json_validate() function, dynamically fetching a class constant, the #[Override] attribute, and more:

• Typed Class Constants
• json_validate() function
• Dynamic class constant fetch
• Marking overridden methods
• mb_str_pad() function
• Fallback value syntax for INI ENV variables
• Support linting multiple files at once
• More Appropriate Date/Time Exceptions
• Deep-cloning of readonly properties
• Randomizer Additions
• and more...

Typed Class Constants
In PHP 8.2, it was still not possible to declare constant (const) types, which can lead to confusion or implications about the type you're working with:
interface I {
const TEST = "Test"; // We may naively assume that the TEST constant is always a string
}
 
class Foo implements I {
const TEST = []; // But it may be an array...
}
 
class Bar extends Foo {
const TEST = null; // Or null
}

Here's an example of how typed constants look in PHP 8.3:
interface I {
const string TEST = E::TEST; // I::TEST is a string as well
}
 
class Foo implements I {
use T;
 
const string TEST = E::TEST; // Foo::TEST must also be a string
}
 
class Bar extends Foo {
const string TEST = "Test2"; // Bar::TEST must also be a string, but the value can change
}

// Error example

// Fatal error: Cannot use array as value for class constant
// Foo::PHP of type string
class Buzz implements I {
const string PHP = [];
}

The json_validate() function
To validate JSON in PHP, you would need to either configure the JSON_THROW_ON_ERROR flag, use json_last_error, or even just check for null on a json_decode() call:
json_decode(json: '{"foo": "bar}', flags: JSON_THROW_ON_ERROR);

// JsonException Control character error, possibly incorrectly encoded.

As of PHP 8.3, you can also validate JSON using the json_validate() function:
// Valid
json_validate('{"framework": "Laravel"}'); // true

// Invalid
json_validate('{"framework": "Laravel}'); // false

json_last_error_msg(); // Control character error, possibly incorrectly encoded
json_last_error(); // 3

Dynamic class constant fetch
In PHP >= 8.2, dynamically fetching a class constant value was only possible using the constant() function. The following would result in a syntax error:
class Framework {
const NAME = 'Laravel';
}

$name = 'NAME';

// You could achieve this with the constant() function
constant(Framework::class . '::' . $name); // Laravel

// This following is a syntax error in >=v8.2.0
echo Framework::{$name};
// ParseError syntax error, unexpected token ";", expecting "(".

As of PHP 8.3, you can now access constants dynamically from a class,
class Framework {
const NAME = 'Laravel';
}

$name = 'NAME';
 
// Syntax error in <= v8.2.0
echo Framework::{$name}; // Laravel

Fallback value for environment variables in INI files
One of my favorite additions to PHP 8.3 is providing a default value when using environment variables to define INI settings. This will simplify Docker defaults without having to specify the defaults as ENV blocks in a Dockerfile. I am sure there are plenty of use cases out there that will be simplified by this.
Let's say, for example, that you want to allow a DRUPAL_FPM_PORT ENV value to configure an INI value for the www FPM pool:
error_log = syslog
daemonize = false

[www]
listen = localhost:${DRUPAL_FPM_PORT}

The DRUPAL_FPM_PORT must be defined, and no default was possible! Now, you can do the following, which should feel familiar to a bash/shell script:
[www]
listen = localhost:${DRUPAL_FPM_PORT:-9000}

Imagine shipping an xdebug.ini file as part of your Dockerfile for development with sensible defaults yet allowing developers to override values they see fit.
Randomizer Additions
PHP 8.3 includes Randomizer Additions, including a new getBytesFromString() method, which is convenient to get random bytes from a source string:
$randomizer = new \Random\Randomizer();

printf(
"%s.example.com",
$randomizer->getBytesFromString('abcdefghijklmnopqrstuvwxyz0123456789', 16)
);

// 3zsw04eiubcf82jd.example.com

// Generate a random code for multi-factor authentication
$randomizer = new \Random\Randomizer(new \Random\Engine\Secure());

echo implode('-', str_split($randomizer->getBytesFromString('0123456789', 20), 5));

// 11551-80418-27047-42075

Learn more
To get up to speed on these new features, check out the PHP 8.3.0 Release Announcement page for examples before/after PHP 8.3. Be sure to check out the deprecations and backward compatibility breaks.

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