<?php

$foo = 1;

function meh(){
  // <-- $foo can't be accessed
}

<?php
$a = 1;
$b = 2;

function Sum()
{
    global $a, $b;

    $b = $a + $b;
}
?>

<?php
$a = 1;
$b = 2;

function Sum()
{
    $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}
?>

<?php

$foo = 1;

function meh(){
  global $foo;
  // <-- $foo now can be accessed
}

?>

function meh(){
  global $foo;
  // $foo now exists in this scope
}