PHP 8.x get variable reference info (call time pass reference)

Question

Call time pass by reference was removed in PHP 5.4. But I have a special case where it seems to be still available in PHP 8 (tried it here: www.w3schools.com):

$myVar = "original";
testFunc([$myVar]);
echo "Variable value: $myVar<br>"; // Output is:  "Variable value: original"
testFunc([&$myVar]);
echo "Variable value: $myVar<br>"; // Output is:  "Variable value: changed"

testFunc([&$undefinedVar]);
echo "Variable value: $undefinedVar<br>"; // Output is:  "Variable value: changed"
testFunc([$undefinedVar_2]);
echo "Variable value: $undefinedVar_2<br>"; // Output is:  "Variable value: "

function testFunc( array $arr ) : void
{
  if ( !is_array($arr)
    || count($arr) == 0 )
    return;
  $arr[0] = 'changed';
}

Additionally, this way I can get a C# like parameter out functionality. Maybe I am misunderstanding something.

Question:
How could I identify within "testFunc" if $arr[0] was passed by reference or normally?

Alternative question (for people who are searching this topic):
Check if variable was passed by reference.

Answer 1

The code by @Foobar pointed me to the right direction. I am using the output of var_dump to analyze it and create a data structure out of it:

class ReferenceInfo
{
    public string $Type;
    public bool $IsReference;
    /** int, float, double, bool are always initialized with default value  */
    public bool $IsInitialized;
    /** @var ?ReferenceInfo[] */
    public ?array $SubItems;

    public static function FromVariable( mixed $arr ) : ?ReferenceInfo
    {
        /** @var ReferenceInfo $rootRefInfo */
        $rootRefInfo = NULL;

        $varInfoStr = self::varDumpToString($arr);
        $varInfoStrArray = preg_split("/\r\n|\n|\r/", $varInfoStr);
        $refInfoObjectStack = [];
        $curKey = NULL;
        foreach ( $varInfoStrArray as $line ) {
            $lineTrimmed = trim($line);
            $lineTrimmedLen = strlen($lineTrimmed);
            if ( $lineTrimmedLen == 0 )
                continue;

            if ( $lineTrimmed == '}' ) {
                array_pop($refInfoObjectStack);
                $curKey = NULL;
                continue;
            }

            if ( $lineTrimmed[0] == '[' ) {
                // Found array key
                $bracketEndPos = strpos($lineTrimmed, ']');
                if ( $bracketEndPos === false )
                    return NULL;
                
                $keyName = self::convertToRealType(substr($lineTrimmed, 1, $bracketEndPos - 1));
                $curKey = $keyName;
                continue;
            }

            $parenPos = strpos($lineTrimmed, '(');
            if ( $parenPos === false ) {
                // Must be a NULL type
                $parenPos = $lineTrimmedLen;
            }

            $type = substr($lineTrimmed, 0, $parenPos);
            $isInitialized = true;
            if ( $type == 'uninitialized' ) {
                $parenEndPos = strpos($lineTrimmed, ')', $parenPos);
                if ( $parenEndPos === false )
                    return NULL;

                $type = substr($lineTrimmed, $parenPos + 1, $parenEndPos - $parenPos - 1);
                $isInitialized = false;
            }

            $refInfoObj = new ReferenceInfo();
            $refInfoObj->IsReference = str_starts_with($type, '&');
            $refInfoObj->IsInitialized = $isInitialized;
            $refInfoObj->Type = substr($type, $refInfoObj->IsReference ? 1 : 0);
            if ( $rootRefInfo == NULL ) {
                $rootRefInfo = $refInfoObj;
            } else {
                $refInfoObjectStack[count($refInfoObjectStack) - 1]->SubItems[$curKey] = $refInfoObj;
            }

            if ( $refInfoObj->Type == 'array'
                || $refInfoObj->Type == 'object' ) {
                $refInfoObj->SubItems = [];
                $refInfoObjectStack[] = $refInfoObj;
            }
        }

        return $rootRefInfo;
    }

    private static function convertToRealType( string $keyName ) : float|int|string
    {
        if ( $keyName[0] == '"' ) {
            $keyName = substr($keyName, 1, strlen($keyName) - 2);
        } else if ( is_numeric($keyName) ) {
            if ( str_contains($keyName, '.') )
                $keyName = doubleval($keyName);
            else
                $keyName = intval($keyName);
        }
        
        return $keyName;
    }

    private static function varDumpToString( mixed $var ) : string
    {
        ob_start();
        var_dump($var);
        return ob_get_clean();
    }
}