Those functions should not return a string when they receive a
non-empty-string in input.
The following example is expected to work:
```php
<?php
/**
* @param non-empty-string $i
*/
function takesNonEmptyString(string $i): void {
echo $i;
}
takesNonEmptyString(bin2hex("a"));
takesNonEmptyString(base64_encode("a"));
```
While there is value in declaring `DateTimeImmutable::createFromInterface()` as mutation-free in
a stub, this method was introduced in PHP 8.0, so we cannot really declare it in a stub.
For now, we drop it, as the value of its stub declaration is much lower than the problems it
introduces through its conditional existence.
Also:
* a non-empty format string will always lead to `non-empty-string`
* it seems that you can throw **everything** at `DateTimeInterface#format()`, even null bytes,
and it will always produce a `string`
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3353-L3369)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3596-L3612)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3308-L3324)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3549-L3565)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3258-L3274)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3496-L3512)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3212-L3228)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3447-L3463)
