How transform a php multidimensional array into a new array using the first column as key?

I'm trying to use an array function (I thought about array_map() or array_walk(), but couldn't get it to do what I want) in order to create an array using a multidimensional array (Like a MySQL result) turning a field from the array into the key of the new one.

Say I have an array like this one:

$a = array(
    0 => array( 'id' => 1, 'name' => 'john' ), 
    1 => array( 'id' => 28, 'name' => 'peter' )
);

And I'd like to get another array like this:

$b = array(
    1 => array( 'name' => 'john' ), 
    28 => array( 'name' => 'peter' )
);

I can solve it with a simple foreach loop, but I wonder whether there's a more efficient way, using a built-in function.

array_map and array_walk don't allow you to change keys. A foreach loop is definitely the way to go. Foreach can even be more efficient than array_walk/array_map a lot of the time.