Given the values
$key = "Main.Sub.SubOfSub";
$target = array();
$value = "SuperData";
Here’s some code I have lying around that does what you need¹:
$path = explode('.', $key);
$root = &$target;
while(count($path) > 1) {
$branch = array_shift($path);
if (!isset($root[$branch])) {
$root[$branch] = array();
}
$root = &$root[$branch];
}
$root[$path[0]] = $value;
¹ Actually it does slightly more than that: it can be trivially encapsulated inside a function, and it is configurable on all three input values (you can pass in an array with existing values, and it will expand it as necessary).