How to sort a multidimensional array by First Name in php?

I've been pulling my hair out for the past 2 hours now. I'm just trying to sort a multidimensional array by firstName. I've been doing a lot of research and can't really find anything similar to my issue :(. Can anyone help me please? Thanks a lot!!

(
[0] => Array
    (
        [userId] => AllieCraig
        [phoneNumber] => 13039453156
        [extension] => 3156
        [lastName] => Craig
        [firstName] => Allie
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )

[1] => Array
    (
        [userId] => 3038003968
        [extension] => 1010
        [lastName] => Abbott_withAReallyLongLastName
        [firstName] => Kim_withAReallyLongFirstNamexx
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )

[2] => Array
    (
        [userId] => 3038003943
        [phoneNumber] => 13038003943
        [extension] => 3943
        [lastName] => Liz
        [firstName] => Voytko
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )

[3] => Array
    (
        [userId] => 3038003970
        [phoneNumber] => 13038003970
        [extension] => 3970
        [lastName] => Spree
        [firstName] => Ed
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )

[4] => Array
    (
        [userId] => 3038003941
        [phoneNumber] => 13038003941
        [extension] => 3941
        [lastName] => Janos
        [firstName] => Ben
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )

[5] => Array
    (
        [userId] => kcypher3969
        [phoneNumber] => 13038003969
        [extension] => 3969
        [lastName] => Cypher
        [firstName] => Keanan
        [externalAccountId] => JimEckles
        [groupName] => JimEckles
        [externalEnterpriseId] => JimEckles
    )
)

via Chebli Mohamed