Add data dynamically to an Array

I want to add data to an array dynamically. How can I do that? Example

$arr1 = [
    'aaa',
    'bbb',
    'ccc',
];
// How can I now add another value?

$arr2 = [
    'A' => 'aaa',
    'B' => 'bbb',
    'C' => 'ccc',
];
// How can I now add a D?