How do I dynamically add elements to arrays in PowerShell?

I don't have much PowerShell experience yet and am trying to teach myself as I go along.

I'm trying to make some proof of concept code for a bigger project. The main goal here is too dynamically create and add elements to an array using a function.

Here is my code:

$testArray = @()
function addToArray($Item1)
{
    $testArray += $Item1
    "###"
}

$tempArray = "123", "321", "453"
$foldertest = "testFolder"

foreach($item in $tempArray)
{
    addToArray $item
}
"###"

Every time the function finishes the array becomes empty. Bear in mind most of my programming experience comes from Java, PHP, some C and C++ just to name a few, if I did this in PHP (adjusting the language syntax of course) this would have worked fine.