Can't understand apply_filter logic

Question

Never creates self filters before. In my plugin I have variable as array. I want another users can change this array without plugin modification. I'm trying this code in my plugin.php:

<?php
/*
Plugin Name: Test plugin
*/

$arr = [
    'val',
    'val2',
    'val3'
];

$arr = apply_filters( 'my_hook', $arr );
print_r( $arr );

In my functions.php I puts this code:

add_filter( 'my_hook', 'modify', 10, 1 );

function modify( $arr ) {
    unset($arr[0]);
    return $arr;
}

I expecting output without first element, but it outputs original array with 3 values;

What's wrong with it?

Answer 1

Thanks for your answers. According to comments above, I modified my code. Now it works. Is it correct way?

<?php
/*
Plugin Name: Test plugin
*/

$arr = [];

add_action( 'init', 'set_var_data' );

function set_var_data() {
    global $arr;

    $arr = [
        'val',
        'val2',
        'val3'
    ];

    $arr = apply_filters( 'my_hook', $arr );
}

print_r( $arr );