We can't do this anymore:

$this->bbcode_uid = substr(base_convert(unique_id(), 16, 36), 0, BBCODE_UID_LEN);  

As of PHP 7.4.0 supplying any invalid characters to base_convert() is deprecated (it only accepts numeric strings). Our unique_id() is creating alpha-numerical strings. The result is the following deprecation errors like:

Deprecated: Invalid characters passed for attempted conversion,

these have been ignored in string(7) "efaoh79"  

 
I have NO idea what bbcode UID's ever were, and I don't even think they are needed anymore with the new bbcode engine. But we certainly need an alternative to this base_convert method.
 
This is being done in the message_parser and functions_convert files.