The secret between the PHP buffer and the header function

In the actual development, have we heard that there is no actual output before the header. Even some think that the header function must be written at the front of the code. But have you ever tried to output something before the header function? Let's get a deeper look at it.

• Test header has output before

     
      I was tested to be successful without any errors or warnings. Don't know what you guys are like? But I think most say is no problem, if there is cannot modify header information-headers already sent such a warning, this is to say can not modify header information, header information has been sent. Come down and find out why there are two different outcomes?

Buffer

As a metaphor, it's like a cache when we watch a movie. It will not be played to us immediately, but the first part of the download of a good movie in the cache, and then the cache play out. And that's how we write PHP code.

PHP's caching mechanism-output_buffering

• Common OB functions in PHP

function	explain
Ob_start	Open output Buffers
Ob_clean	emptying buffers
Ob_get_contents	Return buffer contents
Ob_get_clean	Returns the contents of the buffer and empties

    
 
 

  
  
 
   
In the php.ini configuration file, modify the buffer size
 
 
 
  
 
 
   
 
 

  
  
 
   
Usually around 233 rows, the default is 4096 means 4096 bytes is 4kB
 
  
 
 
   
Down 4096 to 5, rerun this code
   
     
There are no warnings or errors for the test just now, but the error is:cannot modify header information-headers already sent
   
Analysis between header and buffer
   
  
  

   
   
 
    
Why can't we have output before we say header?
 
     
 
      
For the header function, it is like the client sends the original HTTP header, is to declare what we are writing the page exactly what content, so one but this statement before the content is wrong, is not compliant with the HTTP rules
 
     
 
    
Come down and talk about the header in PHP.
 
     
 
      
In PHP, the header is not through the buffer, it will go through the server output directly to the client
 
     
 
    
Explanation of previous warning cannot modify header information
 
     
 
      
When we write some output before the header, it passes through the buffer first. So even if you write the front, the final output sequence is the first header in Echo.
 
      
But the content buffers we output are not fit, that is, the previous output ' Hello world! ' > 5 bytes. will be output directly, that is, the first output ' Hello World ' again header (...), so that it violates the real header can not have output
 
     
 
   
  
  
   
Summarize
   
  
  

   
   
 
    
In practice, we'd better write the header at the front of the page. Because we are not sure if the output before our header is buffer can be dropped.
 
   
  
  
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