Yii 2.0框架源码中，/yiisoft/yii2/base/View.php，看到一个函数，引入php文件，为什么要用ob_start,ob_get_clean？和直接引入文件有什么区别？
<code>/**
* Renders a view file as a PHP script.
*
* This method treats the view file as a PHP script and includes the file.
* It extracts the given parameters and makes them available in the view file.
* The method captures the output of the included view file and returns it as a string.
*
* This method should mainly be called by view renderer or [[renderFile()]].
*
* @param string $_file_ the view file.
* @param array $_params_ the parameters (name-value pairs) that will be extracted and made available in the view file.
* @return string the rendering result
*/
public function renderPhpFile($_file_, $_params_ = [])
{
ob_start();
ob_implicit_flush(false);
extract($_params_, EXTR_OVERWRITE);
require($_file_);
return ob_get_clean();
}</code>直接这样写不也可以
<code>function xx(){
return require($file);
}</code>Yii 2.0框架源码中，/yiisoft/yii2/base/View.php，看到一个函数，引入php文件，为什么要用ob_start,ob_get_clean？和直接引入文件有什么区别？
<code>/**
* Renders a view file as a PHP script.
*
* This method treats the view file as a PHP script and includes the file.
* It extracts the given parameters and makes them available in the view file.
* The method captures the output of the included view file and returns it as a string.
*
* This method should mainly be called by view renderer or [[renderFile()]].
*
* @param string $_file_ the view file.
* @param array $_params_ the parameters (name-value pairs) that will be extracted and made available in the view file.
* @return string the rendering result
*/
public function renderPhpFile($_file_, $_params_ = [])
{
ob_start();
ob_implicit_flush(false);
extract($_params_, EXTR_OVERWRITE);
require($_file_);
return ob_get_clean();
}</code>直接这样写不也可以
<code>function xx(){
return require($file);
}</code>
这段话希望可以帮到你
“Without output buffering, PHP sends data to your web server as soon as it is ready - this might be line by line or code block by code block. Not only is this slow because of the need to send lots of little bits of data, but it also means you are restricted in the order you can send data. Output buffering cures these ills by enabling you to store up your output and send to send it when you are ready to - or to not send it at all, if you so decide.”
ob_get_clean 的意思是先得到 ob_get_contents 的内容，然后再删除缓冲区的内容 (ob_end_clean)。
也就是说：
<code class="php">return ob_get_clean();</code>
等同于：
<code class="php"> $content = ob_get_ob_get_contents(); ob_end_clean(); return $content; </code>
因为直接require会输出文件内容，而这个函数是想返回内容，可以做进一步处理
可能是保证有输出吧,如果使用require,而调用方法之前又有ob_start,那require是没有输出的.
