using varibales as a function name

[eluser]A.M.F[/eluser]
hello,

i am using a foreach loop to show several functions. so i want to do it something like this:

Code:

foreach($fields as $k=>$v)
{
     echo $this->validation->$v[0]._error;
}

but this is the message i get:

Message: Use of undefined constant _error - assumed '_error

why?