fix(php): 解决函数返回引用类型的编译错误

- 添加对函数返回引用类型的检查，禁止返回引用类型
- 修复赋值引用表达式的解析逻辑，确保左侧为变量
- 添加临时变量处理非变量表达式的引用赋值
- 新增函数返回引用的测试用例验证错误处理

phpunit/code/function-return-ref.php

@@ -0,0 +1,15 @@
+<?php
+
+function &test() {
+    global $a;
+    return $a;
+}
+
+function main()
+{
+    $a = null;
+    $b = &test();
+    $b ??= 2;
+
+    var_dump($a, $b);
+}

phpunit/src/FunctionTest.php

@@ -0,0 +1,9 @@
+<?php
+
+class FunctionTest extends \BaseTest
+{
+    public function testReturnRef()
+    {
+        $this->exec('The return type of the function `test` cannot be a reference type', 'function-return-ref.php');
+    }
+}

src/Php/CompilerBase.php

@@ -794,7 +794,11 @@ class CompilerBase extends \PhpAot\Core\Translator
     {
         // .stub 存根定义 C++ Native 函数，必须设置返回值类型
         if (!$v->returnType && $this->stubFile) {
-            throw new \Exception('No return type for ' . $v->name);
+            $this->fatalError($v, 'The return type of the function `' . $v->name . '` must be specified');
+        }
+        // 返回值不能是引用类型
+        if ($v->byRef) {
+            $this->fatalError($v, 'The return type of the function `' . $v->name . '` cannot be a reference type');
         }
 
         $fnName = $this->parseIdentifier($v->name);
@@ -3739,7 +3743,10 @@ class CompilerBase extends \PhpAot\Core\Translator
 
     protected function parseAssignRef(Node\Expr\AssignRef $expr): string
     {
-        if ($this->isVarExpr($expr->var)) {
+        if (!$this->isVarExpr($expr->var)) {
+            $this->fatalError($expr, 'Cannot assign reference to non-variable');
+        }
+
         $this->context->inAssignExpr = true;
         $left = $this->parseIdentifier($expr->var);
         $this->context->inAssignExpr = false;
@@ -3764,8 +3771,10 @@ class CompilerBase extends \PhpAot\Core\Translator
 
             return $left . ' = ' . $object . '.attrRef(' . $prop . ')';
         }
-        }
-        $this->fatalError($expr, 'Cannot assign reference to non-variable');
+
+        $tmpVar = $this->addTmpVar(self::TYPE_VAR);
+        $this->context->beforeStmtLines[] = $tmpVar . ' = ' . $this->parseExpr($expr->expr) . ';';
+        return $left . ' = ' . $tmpVar . '.toReference()';
     }
 
     protected function parseMethodCall(Node\Expr\MethodCall $expr): string