The actual name of the class is
PronouncePHP\Database\Connection\MysqlConnection. Since you've aliased it at the top of the file you can refer to it as
MysqlConnection using literals. That's because literals are fixed in literal scope and name resolution is unambiguous.
However, strings in variables can come from anywhere any time and can hence not realistically be resolved against
use statements. If you want to use the name as a string variable, you need to use the fully qualified name:
$database_class = 'PronouncePHP\Database\Connection\MysqlConnection';