As defined by the RFC, this is totally intentional.

See: https://wiki.php.net/rfc/scalar_type_hints_v5#behaviour_of_weak_type_checks

than this RFC is broken by design because it's completly inconsistent

* native functions: that behavior happens only with declare(strict_types=1)
* useland code: the behavior is ALWAYS like declare(strict_types=1)
_______________________________________________________

OK

[harry@srv-rhsoft:/downloads]$ cat test.php
<?php
 $test = NULL;
 echo trim($test);
?>
[harry@srv-rhsoft:/downloads]$ php test.php
[harry@srv-rhsoft:/downloads]$


[harry@srv-rhsoft:/downloads]$ cat test.php
<?php declare(strict_types=1);
 $test = NULL;
 echo trim($test);
?>
[harry@srv-rhsoft:/downloads]$ php test.php

Fatal error: Uncaught TypeError: trim() expects parameter 1 to be
string, null given in /mnt/data/downloads/test.php:3
Stack trace:
#0 /mnt/data/downloads/test.php(3): trim(NULL)
#1 {main}
  thrown in /mnt/data/downloads/test.php on line 3
________________________________________________________

COMPLETLY BROKEN BEHAVIOR:

[harry@srv-rhsoft:/downloads]$ cat test.php
<?php
 $test = NULL;
 echo test($test);

 function test(string $input)
 {

 }
?>
[harry@srv-rhsoft:/downloads]$ php test.php

Fatal error: Uncaught TypeError: Argument 1 passed to test() must be of
the type string, null given, called in /mnt/data/downloads/test.php on
line 3 and defined in /mnt/data/downloads/test.php:5
Stack trace:
#0 /mnt/data/downloads/test.php(3): test(NULL)
#1 {main}
  thrown in /mnt/data/downloads/test.php on line 5

and it defeats the whole purpose because before you just wrote

function($x)
{
 $x = (int)$x;
}

now with function(int $x) you must place the (int)-casting all around your code in the callers or check explicit if the value is NULL