Description:
------------
max() throws a warning when given an empty array. The 
documentation reads:

"If the first and only parameter is an array, max() returns 
the highest value in that array. ... You can compare an 
unlimited number of values."

"Unlimited number of values", by definition of unlimited, 
includes zero values. (If there aren't any values in the 
array, the highest value is the value null.) This is 
consistent with other max() implementations -- MySQL, Oracle, 
Postgres, Ruby...

Please either remove the warning and return null, or update 
the documentation to reflect the existence of the warning.

Reproduce code:
---------------
<?

$a = array(1,3,2);
function foo() {
  global $a;
  $v = max($a);
  echo is_null($v) ? "(null)\n" : "$v\n";
  array_pop($a);
}

foo($a);
foo($a);
foo($a);
foo($a);

?>

Expected result:
----------------
3
3
1
(null)

Actual result:
--------------
3
3
1
PHP Warning:  max(): Array must contain atleast one element in 
foo.php on line 6