php foreach with multidimensional array

You can use foreach here just fine.

foreach ($rows as $row) {
    echo $row['id'];
    echo $row['firstname'];
    echo $row['lastname'];
}

I think you are used to accessing the data with numerical indicies (such as $row[0]), but this is not necessary. We can use associative arrays to get the data we're after.

You can use array_walk_recursive:

array_walk_recursive($array, function ($item, $key) {
    echo "$key holds $item\n";
});

This would have been a comment under Brad's answer, but I don't have a high enough reputation.

Recently I found that I needed the key of the multidimensional array too, i.e., it wasn't just an index for the array, in the foreach loop.

In order to achieve that, you could use something very similar to the accepted answer, but instead split the key and value as follows

foreach ($mda as $mdaKey => $mdaData) {
    echo $mdaKey . ": " . $mdaData["value"];
}

Hope that helps someone.