PHP mb_convert_encoding turns ‘blue’ into ‘扬略’

I’m encountering a strange behavior when using mb_convert_encoding in PHP.
I have a variable $input containing the string ‘blue’, and when I try to convert it to UTF-8 using the following code:

$input = 'blue';
$str = mb_convert_encoding((string)$input, 'UTF-8', mb_list_encodings());

The resulting value of $str is ‘扬略’ instead of ‘blue’.
Can someone please help me understand what might be causing this unexpected transformation and how I can resolve it?