php

php vendor/bin/phpcs -p ./web/modules/custom/ --standard=PHPCompatibility --extensions=module/php,inc/php,install/php,php/php --runtime-set testVersion 8.4

$new_date = date('d.m.Y', strtotime($existing_date));

$flat_tags = array_diff($flat_tags, [$favorites_tag->id()]);

$tags = $form_state->getValue('tags');
$flat_tags = [];
array_walk_recursive($tags, static function ($a) use (&$flat_tags) {
  $flat_tags[] = $a;
});

// Or simple:

$flat_tags = array_column($tags, 'target_id');