support empty/isset with undefined var

src/CompilerBase.php

@@ -3228,8 +3228,15 @@ class CompilerBase implements PropertyAccessContext
 
     protected function parseChainedExpr(NodeAbstract $node, string $op, bool $getValue = false): string
     {
+        $name = $this->parseIdentifier($node);
+
+        // 如果empty/isset中的变量不存在，直接返回true/false
+        if (($op == self::OP_EMPTY || $op == self::OP_ISSET) && $this->isVarExpr($node) && !$this->hasVar($name)) {
+            return $op == self::OP_EMPTY ? 'true' : 'false';
+        }
+
         // TypePHP 编译器不允许操作未定义的变量，PHP 的 isset($var) 可能 $var 未定义
-        $this->checkVarMustExist($node, $this->parseIdentifier($node));
+        $this->checkVarMustExist($node, $name);
         $fn = $this->getChainedFunc($op);
         $expr = $node;
         if ($this->isVarExpr($expr)) {

tests/std/core/empty_isset.phpt

@@ -0,0 +1,16 @@
+--TEST--
+empty() / isset() undefined var
+--FILE--
+<?php
+function main() {
+    var_dump(empty($do));
+    var_dump(isset($do));
+    var_dump(!empty($do));
+    var_dump(!isset($do));
+}
+?>
+--EXPECT--
+bool(true)
+bool(false)
+bool(false)
+bool(true)