Closes #1937

Updated /packages/view/src/Elements/PhpIfElement.php

     public function compile(): string
     {
         if ($condition = $this->wrappingElement->consumeAttribute(':isset')) {
-            $condition = sprintf('isset(%s)', $condition);
+            $condition = preg_replace('/\$\w+/', 'isset($0)', $condition);

Updated docs and tests, and real-world tested it in an app, both in single-case and multi-case.

Expression logic is untested, because all the function does is wrap isset around a $var, so it remains the responsibility of a developer to produce expressions which evaluate to true or false as needed.