Accessing variable inside function - php

Accessing variable inside function - php

i need to access a variable which is declared in another php file within a
function.. How can i do it?
a.php
<?php
$global['words']=array("one","two","three");
echo "welcome"
?>
b.php
<?php
$words = $global['words'];
require_once('a.php');
print_r($global['words']);
function fun()
{
print_r($global['words']); // not displaying here
}
fun();
?>
now i am able to access the "$global['words']" variable in b.php file, but
not within function, how can i make it visible inside the function?