VAriable value

I’m new to wordpress and php beginner and I’m learning how to debug the code. I’m building a widget and I want to see the value of a variable but I don’t know how cause echo and var_dump won’t output anything inside the widget.

add_filter(‘filter’, myCallBack);

function myCallBack($content){
//here I would like to see the value of $content but
var_dump($content); //nothing outputted
echo $content; //nothing here either
return $content
}

Thanks in advance for the help.