Getting the key of the only element in a PHP array

The key of the associative array is dynamically generated. How do I get the "Key" of such an array?

$arr = array ('dynamic_key' => 'Value');

I am aware that It is possible to access it through a foreach loop like this:

foreach ($arr as $key => $val) echo "Key value is $key";

However, I know that this array will have only one key and want to avoid a foreach loop. Is it possible to access the value of this element in any other way? Or get the key name?