Use array_slice....
http://docs.php.net/manual/da/function.array-slice.php
Example code
$array = array(
'cat' => 'Tabby',
'dog' => 'Jack Rusell',
'bear' => 'Brown',
'horse' => 'Mustang',
);
$result = array_slice($array, 0, 2, true) +
array("tiger" => "Bengal") +
array_slice($array, 2, count($array) - 1, true) ;
echo '<pre>',print_r($result),'</pre>';
Results shown below
Array
(
[cat] => Tabby
[dog] => Jack Rusell
[tiger] => Bengal
[bear] => Brown
[horse] => Mustang
)