Tri Array après array_merge

Bonjour,
J'ai un soucis de tri de deux tableaux réunis en un seul avec une fonction array_merge.
Le but étant d'afficher une liste amis par ordre alphabétique du prénom obtenu avec 2 requêtes de 2 manières différentes.
Grâce a des requêtes SQL j'obtiens 2 tableaux $friend1 et $friend2.
Les tableaux sont tous les deux classés par ordre alphabétique du firstname grâce à un ORDER BY.

print($friend1) :

Array (
[0] => Array ( [0] => 3 [id_friend] => 3 [1] => 2 [id_gender] => 2 [2] => 3 [id_customer] => 3 [3] => LUMIERE [lastname] => LUMIERE [4] => Barbapapa [firstname] => Barbapapa )
[1] => Array ( [0] => 11 [id_friend] => 11 [1] => 2 [id_gender] => 2 [2] => 11 [id_customer] => 11 [3] => DRAPEAUX [lastname] => DRAPEAUX [4] => Cloche [firstname] => Cloche )
[2] => Array ( [0] => 19 [id_friend] => 19 [1] => 2 [id_gender] => 2 [2] => 19 [id_customer] => 19 [3] => BOEUF [lastname] => BOEUF [4] => Fichtre [firstname] => Fichtre )
[3] => Array ( [0] => 2 [id_friend] => 2 [1] => 1 [id_gender] => 1 [2] => 2 [id_customer] => 2 [3] => HIBOUX [lastname] => HIBOUX [4] => Grotte [firstname] => Grotte )
[4] => Array ( [0] => 17 [id_friend] => 17 [1] => 2 [id_gender] => 2 [2] => 17 [id_customer] => 17 [3] => LUNE [lastname] => LUNE [4] => Jasmin [firstname] => Jasmin )
)

print($friend2) :

Array (
[0] => Array ( [0] => 10 [id_customer] => 10 [1] => 2 [id_gender] => 2 [2] => 10 [3] => LOUTRE [lastname] => LOUTRE [4] => Arbre [firstname] => Arbre )
[1] => Array ( [0] => 18 [id_customer] => 18 [1] => 1 [id_gender] => 1 [2] => 18 [3] => CANARD [lastname] => CANARD [4] => Rocher [firstname] => Rocher )
[2] => Array ( [0] => 12 [id_customer] => 12 [1] => 2 [id_gender] => 2 [2] => 12 [3] => BICHETTE [lastname] => BICHETTE [4] => Saucisse [firstname] => Saucisse )
)

Avec un $res=array_merge((array)$friend2,(array)$friend1) j'obtient:

print($res) :

Array (
[0] => Array ( [0] => 3 [id_friend] => 3 [1] => 2 [id_gender] => 2 [2] => 3 [id_customer] => 3 [3] => LUMIERE [lastname] => LUMIERE [4] => Barbapapa [firstname] => Barbapapa )
[1] => Array ( [0] => 11 [id_friend] => 11 [1] => 2 [id_gender] => 2 [2] => 11 [id_customer] => 11 [3] => DRAPEAUX [lastname] => DRAPEAUX [4] => Cloche [firstname] => Cloche )
[2] => Array ( [0] => 19 [id_friend] => 19 [1] => 2 [id_gender] => 2 [2] => 19 [id_customer] => 19 [3] => BOEUF [lastname] => BOEUF [4] => Fichtre [firstname] => Fichtre )
[3] => Array ( [0] => 2 [id_friend] => 2 [1] => 1 [id_gender] => 1 [2] => 2 [id_customer] => 2 [3] => HIBOUX [lastname] => HIBOUX [4] => Grotte [firstname] => Grotte )
[4] => Array ( [0] => 17 [id_friend] => 17 [1] => 2 [id_gender] => 2 [2] => 17 [id_customer] => 17 [3] => LUNE [lastname] => LUNE [4] => Jasmin [firstname] => Jasmin )
[5] => Array ( [0] => 10 [id_customer] => 10 [1] => 2 [id_gender] => 2 [2] => 10 [3] => LOUTRE [lastname] => LOUTRE [4] => Arbre [firstname] => Arbre )
[6] => Array ( [0] => 18 [id_customer] => 18 [1] => 1 [id_gender] => 1 [2] => 18 [3] => CANARD [lastname] => CANARD [4] => Rocher [firstname] => Rocher )
[7] => Array ( [0] => 12 [id_customer] => 12 [1] => 2 [id_gender] => 2 [2] => 12 [3] => BICHETTE [lastname] => BICHETTE [4] => Saucisse [firstname] => Saucisse )
)

Le PB est que le tableau n'est plus classé par ordre alphabétique puisque le Array_merge rajoute simplement l'un après l'autre.
J'ai vu qu'il y avait des fonctions de tri pour les tableaux simple mais ne vois pas trop comment les appliquer pour le type de tableau que j'obtient.