How to get variables from fucntion.php to my plugin files

There is three way to use function.php file variable into plugin file.

1. Using global. Make sure you globalize it first.

   global $my_variable;
echo $my_variable;

2. I recommend is using WordPress built-in filter mechanism add_filter. You add the filter in your functions.php file and apply it where needed.

In functions.php:

add_filter( 'my_variable', 'return_my_variable' );
function return_my_variable( $arg = '' ) {
    return '111221122';
}

Now you can use in your plugin files:

echo apply_filters( 'my_variable', '' );

3. Use an action hook add_action

In functions.php:

add_action( 'my_variable', 'echo_my_variable' );
function echo_my_variable() {
    echo '888998899';
}

In your plugin files:

do_action( 'my_variable' );

Again, I recommend a filter because it can return a value. This is far more flexible than injecting echo calls in your plugin file. Whether you use an action hook or a filter is entirely up to you.