Global Scope
The global scope spans all included files. That is, $foo will be visible in file.php. 

<?php
    $foo = 1;
    include 'file.php';
?>
Function Local Scope
If you wish to access a global variable from within a function, you have to use the global keyword to tell the function that it should use the variable from the global scope instead of the function's own local scope. 

<?php
function bar() {
    global $foo;

    echo $foo;
}

$foo = 1;
bar();
?>
Output
1
Static Variables
<?php
function bar2() {
    static $i=0;
    return ++$i;
}
echo bar2()."<br>\n";
echo bar2()."<br>\n";
?>
Output
1
2