How does PHP append one more key-value pair to the end of the key-value pair array?

How does PHP append one key-value pair to the end of the key-value pair array? How does PHP append another key-value pair to the end of the key-value pair array?
For example
$ Arr1 = array (
"A" => '1 ',
"B" => '2 ',
"C" => '3 ',
"D" => '4 ');
Convert it
$ Arr2 = array (
"A" => '1 ',
"B" => '2 ',
"C" => '3 ',
"D" => '4 ',
"F" => '5 ',);
What to do. To add a new key-value pair to the array.

Reply to discussion (solution)

Can I assign values directly !!! So easy

$ Array ['F'] = $ array;