php - Why I can't use get_cfg_var() to assign a custom setting value to an instance variable

I am
doing the
following:

class
            JConfig {
 public $var =
            get_cfg_var('mySetting');
}
?>

If
I do that I received a compile error that says:

syntax
error, unexpected '(', expecting ',' or
';'

If I do
echo get_cfg_var('mySetting') outside the class it works just
fine.
If I assign get_cfg_var('mySetting') to a
local variable it works just fine.

Anyone has
any idea why I can't assign it to a instance variable?