global konusuna ufak bir örnek yapalım

<?php
$a = 5;
$b = 20;
function topla(){
    global $a, $b, $c, $d, $e, $f;
        $c = $a * $b;
        $d = $a + $b;
        $e = $a / $b;
        $f = $a - $b;
}
topla();
  echo "$a * $b = $c"."<br>";
  echo "$a + $b = $d"."<br>";
  echo "$a / $b = $e"."<br>";
  echo "$a - $b = $f"."<br>";
?>