PHP - Get key name of array value

Question

I have an array of the following:

function example() {
    /* some stuff here that pushes items with
        dynamically created key strings into an array */

    return array( // now lets pretend it returns the created array
        'firstStringName' => $whatEver,
        'secondStringName' => $somethingElse
    );
}

$arr = example();

// now I know that $arr contains $arr['firstStringName'];

In order to loop through array keys($arr) and return the key string "firstStringName" by its index, I need to get the index of $arr['firstStringName']. How can I go about doing that?