add values to array dynamically php

There are quite a few ways to work with dynamic arrays in PHP. Initialise an array:

$array = array();
Add to an array:

$array[] = "item"; // for your $arr1 
$array[$key] = "item"; // for your $arr2
array_push($array, "item", "another item");
Remove from an array:

$item = array_pop($array);
$item = array_shift($array);
unset($array[$key]);
There are plenty more ways, these are just some examples.