Corrección de mensaje Warning: Cannot modify header information

Warning: Cannot modify header information - headers already sent by (output started at /hermes/walnaweb01a/b676/moo.vickuscom/mvc/libs/Controller.php:12) in /hermes/walnaweb01a/b676/moo.vickuscom/mvc/models/login_model.php on line 35

This is a message sended from a web PHP page were the code

header(Location:../dashboard);

was scripted to call another page of the web. but the message of the top was showed, to resolve this, All the messages for debug were commented or eliminated, like this one, including one that sends the message to the console, trying to evade the error.

echo("<script>console.log('PHP: Controller.loadModel ".$path."');</script>");
echo "UPDATE $table SET $fieldDetails WHERE $where";

Now the question is, how to get debug messages ...