Scan for and fix any PHP 7.4+ compatibility issues

[johnstray]

Scan all the code for any php 7.4+ & 8.x incompatibilities, such as depreciated functions, etc.

• mb_convert_encoding() - Handling HTML entities via mbstring is depreciated - multiple locations
• str_replace(): Passing null to parameter # 3 ($subject) of type array|string is deprecated in /admin/inc/basic.php on line 2902