Regarding "Neither functions nor constants can be imported via the use statement." Actually you can do it in PHP 5.6+:
<?php
// importing a function (PHP 5.6+)
use function My\Full\functionName;
// aliasing a function (PHP 5.6+)
use function My\Full\functionName as func;
// importing a constant (PHP 5.6+)
use const My\Full\CONSTANT;
?>