Idea for having default instances of a class available, lazy-loaded.

currently-in.php

<?php
final class Timezone
{
    private static $utc;
    
    public static function utc() : self
    {
        return self::$utc ?: self::$utc = new self('utc');
    }
}

// Each time you need the UTC timezone, PHP needs to make a function call.
$utc = TimeZone::utc();

in-eg.java

final class Timezone
{
    final public static utc = new Timezone('utc');
}

maybe-in.php

<?php
final class Timezone
{
    public const UTC = new self('utc');
}

// The idea here is rather simple. A constant reference to an object is stored as constant.
// Anything which would be required for this is to allow "new ClassName" constructs as
// constant expression. The value, which is the reference to an object, would actually be
// constant and thus not changable.
//
// PHP wouldn't need to take any more special care about the object in question, as it is
// its own responsibility to be immutable if required. This would not only reduce the
// execution time for that case, but also reduce the amount of code (and logic) required,
// which can become quite a lot if you need multiple default instances.
//
// Since constant expressions are only evaluated once the constant is accessed for the first
// time, this would additionally benefit from automatic lazy loading.
$utc = Timezone::UTC;