In versions prior to PHP 5.2.2 offsetExists() would return false if index value is null.
<?php
// running PHP 5.2.1
$params = new ArrayObject(array('INT'=>null, 'STR'=> null, 'BOOL'=>null, 'LOB'=>null));
$test = $params->offsetExists('INT');
var_dump($test);
// result would be bool(false)
// running PHP 5.2.2
$params = new ArrayObject(array('INT'=>null, 'STR'=> null, 'BOOL'=>null, 'LOB'=>null));
$test = $params->offsetExists('INT');
var_dump($test);
// result would be bool(true)
?>
Both tests where made on Windows platform.