<?php
  $x=5; // global scope

  function myTest()
  {
  echo $x; // local scope
  }

  myTest();
?>

A variable that is defined outside of any function, has a global scope.
Global variables can be accessed from any part of the script, EXCEPT from within a function.
To access a global variable from within a function, use the global keyword:

Example

<?php
$x=5; // global scope
$y=10; // global scope

function myTest()
{
global $x,$y;
$y=$x+$y;
}

myTest();
echo $y; // outputs 15
?>

PHP also stores all global variables in an array called $GLOBALS[index]. The index holds the name of the variable. This array is also accessible from within functions and can be used to update global variables directly.

The example above can be rewritten like this:

Example

<?php
$x=5;
$y=10;

function myTest()
{
$GLOBALS['y']=$GLOBALS['x']+$GLOBALS['y'];
}

myTest();
echo $y;
?>

<?php
function test() {
$foo = "local variable";

echo '$foo in global scope: ' . $GLOBALS["foo"] . "\n";
echo '$foo in current scope: ' . $foo . "\n";
}

$foo = "Example content";
test();
?>