If you want to append null values wrap them in an array:
<?php
$a = array('Hey', 'hey', 'my', 'my');
array_splice($a, 1, 0, null);
print_r($a);
?>
Array
(
[0] => Hey
[1] => hey
[2] => my
[3] => my
)
<?php
$b = array('Hey', 'hey', 'my', 'my');
array_splice($b, 1, 0, array(null));
print_r($b);
?>
Array
(
[0] => Hey
[1] =>
[2] => hey
[3] => my
[4] => my
)