Fixes yiisoft#15046

framework/CHANGELOG.md

@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.14 under development
 ------------------------
 
+- Enh #15046: Throw an `yii\web\HeadersAlreadySentException` if headers were sent before web response
 - Bug #15355: Fixed `yii\db\Query::from()` does not work with `yii\db\Expression` (vladis84)
 - Bug #8983: Only truncate the original log file for rotation (matthewyang, developeruz)
 - Bug #14276: Fixed I18N format with dotted parameters (developeruz)

framework/web/HeadersAlreadySentException.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * @link http://www.yiiframework.com/
+ * @copyright Copyright (c) 2008 Yii Software LLC
+ * @license http://www.yiiframework.com/license/
+ */
+
+namespace yii\web;
+
+use yii\base\Exception;
+
+/**
+ * HeadersAlreadySentException represents an exception caused by
+ * any headers that were already sent before web response was sent.
+ *
+ * @author Dmitry Dorogin <dmirogin@ya.ru>
+ * @since 2.0.14
+ */
+class HeadersAlreadySentException extends Exception
+{
+    /**
+     * Create instance of exception depending on debug mode.
+     *
+     * @param string $file
+     * @param int $line
+     * @return HeadersAlreadySentException
+     */
+    public static function make($file, $line)
+    {
+        $message = YII_DEBUG ? "Headers already sent in {$file} on line {$line}." : 'Headers already sent.';
+        return new static($message);
+    }
+}

framework/web/Response.php

@@ -362,8 +362,8 @@ public function clear()
      */
     protected function sendHeaders()
     {
-        if (headers_sent()) {
-            return;
+        if (headers_sent($file, $line)) {
+            throw HeadersAlreadySentException::make($file, $line);
         }
         if ($this->_headers) {
             $headers = $this->getHeaders();