Error in dev mode

[Carlos S.]

Every time I enable dev mode on my test site I get this error 

define( 'IN_DEV', TRUE );
Fatal error: Uncaught exception 'ErrorException' with message 'Cannot modify header information - headers already sent by (output started at /home2/moomingaming/public_html/test/constants.php:1)' in /home2/moomingaming/public_html/test/init.php:542 Stack trace: #0 [internal function]: IPS\IPS::errorHandler(2, 'Cannot modify h...', '/home2/moominga...', 542, Array) #1 /home2/moomingaming/public_html/test/init.php(542): header('HTTP/1.1 500 In...') #2 /home2/moomingaming/public_html/test/init.php(517): IPS\IPS::genericExceptionPage('session_start()...') #3 [internal function]: IPS\IPS::exceptionHandler(Object(ErrorException)) #4 {main} thrown in /home2/moomingaming/public_html/test/init.php on line 542

What is wrong? I followed all the steps on the guide

[MADMAN32395]

and you uploaded the dev tools?

 

[Carlos S.]

10 hours ago, MADMAN32395 said:

and you uploaded the dev tools?

 

yes