Smarty v5 update test, initial code setup

src/Compile/Modifier/IssetModifierCompiler.php

@@ -0,0 +1,25 @@
+<?php
+namespace Smarty\Compile\Modifier;
+use Smarty\CompilerException;
+
+/**
+ * Smarty isset modifier plugin
+ */
+class IssetModifierCompiler extends Base {
+
+	public function compile($params, \Smarty\Compiler\Template $compiler) {
+
+		$params = array_filter($params, function($v) { return !empty($v); });
+
+		if (count($params) < 1) {
+			throw new CompilerException("Invalid number of arguments for isset. isset expects at least one parameter.");
+		}
+
+		$tests = [];
+		foreach ($params as $param) {
+			$tests[] = 'null !== (' . $param . ' ?? null)';
+		}
+		return '(' . implode(' && ', $tests) . ')';
+	}
+
+}