How can I define a local variable in a function in PHP?

Assuming I understand what you're asking for here, you could do this:

class Private {

    private $user = 'value'; // Declare $user as a private variable

    function ha() {

        echo $this->user;

    }

}

$object = new Private();

echo $object->user; // Returns fatal error
$object->ha(); // Echoes 'value'

More on visibility is in the PHP documentation.

You are asking about defining a local variable, but that by itself would not solve your problem.
For the sake of those getting here by a search, I'll answer both.

• How to define a local variable in a function:

PHP defines a variable on its first use. There is no keyword for declaring a local scope. All variables inside functions are local by default (even a variable with the same name as another global variable).

'A first use' means assigning a value, not using the variable in return or a condition. If you are not certain that the variable you use on return will be assigned a value (e.g. inside a config.php file), then you need to initialize it using a value of desired type. e.g.: $user = ''.

• import an external config.php file inside a local scope of a function

You wanted to:

Define a local variable inside a function, accessible only inside that function's scope. This local variable $user is assigned a value from the config.php file.
$user must not be visible from outside of that function.

You need to put the require_once statement inside that function.

If the include occurs inside a function within the calling file, then all of the code contained in the called file will behave as though it had been defined inside that function. http://php.net/manual/en/function.include.php

function ha(){
  static $user;
  require_once('../config.php');
  if (!isset($user)) { $user = ''; } // Only needed if you are not sure config.php will define the $user variable
  return $user;
}

echo ha();

echo $user; // Will output nothing

Using static keyword will let the function keep the value of $user. This is needed, because if you call the function a second time, file config.php will not be included again (require_once()).

PHP has three variable scopes, global, local, and static. Static is not the same as local, but it behaves the same in a sense to where the variable is accessible. Static scope in a function is a PHP specialty, read about it.

when I define $user in function static I get an empty value..

You do, because when you used require_once() outside of the function, $user variable was defined in global scope. Then you defined another $user variable inside the ha function by using $user, without declaring it to be global by global $user, as you did later.

require_once('../config.php');

function ha(){
    $user; /* THIS I NEED TO DEFINE LOCAL ONLY ACCESS FROM THIS FUNCTION */
    return $user;
}

echo ha();

//echo $user; /* (unnecessary) GLOBAL VARIABLE VALUE, same as echoing the result of the function!*/

Please see the documentation on variable scope. The second echo $user is now unnecessary. Any variable declared in a function not explicitly set to be $global will not be global.