get_template_part() vs. require - variable scope

1-template.php

<?php
$hello = 'world';

get_template_part( 'hello' );

require 'hello.php';

function get_template_part( $filename ) { 
    require $filename . '.php';
}

2-hello.php

Hello, <?php echo $hello ?>

3-The output

Hello, 
Hello, world