$arr1 = ['a'=>1, 'b'=>2, 'c'=>3];
$arr2 = $arr1;

unset($arr2['a'];
print_r($arr1);
//['b'=>2, 'c'=>3]
// ------- OR -------
$arr2 = clone $arr1;

unset($arr2['a'];
print_r($arr1);
//['a'=>1, 'b'=>2, 'c'=>3]