If you want to know how to notate a Unicode character like U+1F606 in PHP, then do this:
$myChar = "\u{1F606}";
You must write it down like this and not make it up from substrings.
If you have '1F606' as a character string, you must convert it.
$code = "1F606";
$myChar = html_entity_decode('&#x'.$code.";", ENT_QUOTES, 'UTF-8');
Demo: https://3v4l.org/312KC
Variant 3: You can also write the emjois directly into your code.
$myChar = "";
The bin2hex
function supplies the individual hexadecimal bytes.
$hex = bin2hex($myChar); //"f09f9886"