PHP sort an array of arrays according to an index in the arrays

Question

Let's say you have an array that looks like this:

$myArray []= array('firstname' => 'John' , 'lastname'=> 'Johnson');

$myArray []= array('firstname' => 'Adam' , 'lastname'=> 'Tyson');

$myArray []= array('firstname' => 'Mike' , 'lastname'=> 'Robinson');

$myArray []= array('firstname' => 'David' , 'lastname'=> 'Jackson');

How can you sort $myArray according to the lastname?