$a=1;
function $test(){
echo $a;
}

//outputs 1

$test = array(
0=>'zero',
1=>'one',
2=>'two',
3=>'three',
);

function doesntWork($something){
echo "My favorite number is " . $test[$something];
}

//outputs My favorite number is 0

<?php
  $a=1;
  function test($foo) {
    echo 'number ' . $foo;
  };

  test($a);
  // -> "number 1".
 ?>

<?php
  $test = array(
    0=>'zero',
    1=>'one',
    2=>'two',
    3=>'three',
  );

  function doesntWork($test, $something){
    echo "My favorite number is " . $test[$something];
  }

  doesntWork($test, mt_rand(0,3));
?>

function test() {
    global $a;
    echo $a;
}

function doesWork($something) {
    global $test;
    echo "My favorite number is " . $test[$something];
}