filter_var with FILTER_VALIDATE_REGEXP shows Warning: filter_var(): 'regexp' option missing, while this option seems to be present

Question

Background

I'd like to use filter_var() with FILTER_VALIDATE_REGEXP to validate a UUID Version 4 value. I don't want to use preg_match(). I've been searching and testing for hours, and can't get rid of the following problem.

Input variables

I have three variables:

• $value: contains the value submitted by the client/browser
• $filterType: is FILTER_VALIDATE_REGEXP before the code below is executed (otherwise the part of the code isn't executed)
• $filterOption: filteroptions, read from a json configuration file before the code below is executed

This is how the config file is read and converted into json:

$file = dirname(__FILE__).'/'.CONFIG_FILE_QUERYPARAMETER;
if (file_exists($file)) {
   $json = file_get_contents($file);
   if (false !== $json) {
      $config = json_decode($json, false);

Code

$filterOptionConst is just defined and used as comparison/working example, and shall be removed when the problem is identified.

$filterOptionConst = array('options' => array('regexp' => '/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/'));

var_dump($filterOption); echo '<br />';       // line 3
var_dump($filterOptionConst); echo '<br />';  // line 4
var_dump($filterOptionConst === $filterOption); echo '<br />'; // line 5

$value1 = filter_var($value, $filterType, $filterOption);           // line 7, warning
$value2 = filter_var($value, $filterType, $filterOptionConst);      // line 8, no warning
$value3 = filter_var($value, $filterType, array('options' => $filterOption['options'])); // line 9, warning
$value4 = filter_var($value, $filterType, array('options' => array('regexp' => $filterOption['options']['regexp'])));  // line 10, no warning

Output

Both line 3 and 4 show the sames output (copy-pasted from actual output; I can't see the difference, at least). So $filterOption and $filterOptionConst should have identical values.

array(1) { ["options"]=> array(1) { ["regexp"]=> string(71) "/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/" } }
array(1) { ["options"]=> array(1) { ["regexp"]=> string(71) "/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/" } }
bool(true)

But then I get:

Warning: filter_var(): 'regexp' option missing in (filename) on line 7
Warning: filter_var(): 'regexp' option missing in (filename) on line 9

What makes me crazy

Lines 7 and 8 call the same function with the same arguments (?), and line 7 throws a warning while line 8 doesn't. The order of the lines (from 7 to 10) don't matter.

Does anyone have any clue what might happen here? Why do line 7 and 9 cause a warning, while 8 (and 10) don't do so? Of course, I'd like to go with line 7, as I use the filter options also for other filters like FILTER_VALIDATE_INT wihout manipulating them. So, the desired result would be a line 7 without warning.

Any help is highly appreciated!

Update 27.02.2022 (a)

I need to import the configuration file as object structure, so that I can clearly distinct between objects and (indexed) arrays. Thus, I have a function that recursively converts the filter options from objects to associative arrays. The function is tested and seemingly delivers the right output, but somehow filter_var doesn't handle the (equal?) input equally.

function object2array($obj) {
   // only process if it's an object or array being passed to the function
   if (is_object($obj) || is_array($obj)) {
      $result = (array) $obj;
      foreach($result as &$item) {
         //recursively process EACH element regardless of type
         $item = object2array($item);
      }
      return ($result);
   }
   //otherwise (i.e. for scalar values) return without modification
   else {
      return ($obj);
   }
}

So, this works,

$filteroptions = array('regexp' => $configuration->filter->options->regexp);

this works as well,

$filteroptions = (array) $configuration->filter->options;

and this - with the 'same' output leads to the warning when using filter_var() later on.

$filteroptions = object2array($configuration->filter->options);

I will further investigate this and update here. Of course, if anyone sees the difference, any hint is still highly appreciated.

Update 27.02.2022 (b)

I've found the issue, but don't know why this is a problem. The warning disappers when I remove the & in the foreach in object2array(). The following code (loop without content!) already results in the warning later on.

foreach($result as &$item) {
}

So, I changed

foreach($result as &$item) {

to

foreach($result as $item) {

Of course, I need to change the item, so I changed the loop to

foreach($result as $key => $item) {
   $result[$key] = object2array($item);
}

Now, I don't have any warnings. But I still don't know why the warnings occured. Does anyone have any hint? Could this be a bug in PHP? Is there any way to debug arrays/objects beyond var_dump()?