array_splice() does not preserve numeric keys. The function posted by "weikard at gmx dot de" won't do that either because array_merge() does not preserve numeric keys either.

Use following function instead:

<?php
function arrayInsert($array, $position, $insertArray)
{
    $ret = [];

    if ($position == count($array)) {
        $ret = $array + $insertArray;
    }
    else {
        $i = 0;
        foreach ($array as $key => $value) {
            if ($position == $i++) {
                $ret += $insertArray;
            }

            $ret[$key] = $value;
        }
    }

    return $ret;
}
?>

Example:
<?php
$a = [
    295 => "Hello",
    58 => "world",
];

$a = arrayInsert($a, 1, [123 => "little"]);

/*
Output:
Array
(
    [295] => Hello
    [123] => little
    [58] => world
)
*/
?>

It preserves numeric keys. Note that the function does not use a reference to the original array but returns a new array (I see absolutely no reason how the performance would be increased by using a reference when modifying an array through PHP script code).