Why is an error reported in session_start () after the button label is added?

If the button label in the navigation bar is added, an error is returned. if the button label is removed, the following error message is displayed: Warning: session_start (): Cannotsendsessioncachelimiter. when the button label in the navigation bar is added, an error is returned.
Warning: session_start (): Cannot send session cache limiter-headers already sent
Is there output in front of session_start? Why is there a p error? Why is there a label a in the navigation bar? But when the button label is added, an error is reported?

  
   
 
   
   W3Cschool 
   
   
 
  
   
  
  
aaaaa
  

Reply content:

When the button label in the navigation bar is added, an error is reported. if the button label is removed, the following error is returned:
Warning: session_start (): Cannot send session cache limiter-headers already sent
Is there output in front of session_start? Why is there a p error? Why is there a label a in the navigation bar? But when the button label is added, an error is reported?

  
   
 
   
   W3Cschool 
   
   
 
  
   
  
  
aaaaa
  

You putsession_start()Put it at the beginning.
The button will have output, but no output can be provided before the HTTP cookie request, even if it is a blank line.

Put session_start () on the page header ()

I use php5.5 locally. no error is reported when using your code.