Getting a record set from associative array

Hello

I am dealing with a problem and having a hard time solving it. So need your help!

Imagine I have a comment section in my website, where there are two types of role “member” and “staff”. Both roles can post comment. I want to get the datetime of the [B]first[/B] post made by the [B]opposite[/B] role. For example, in the following array the latest post was made by “aaa” at 1433566756. So, how do I programitically get the first post made by the opp role (i.e. post made by ccc at 1433566752)

Note: The following array is in reverse chronological order. Meaning, the latest post appears on top. Also, to make it less confusing, I have used unique datetime for each record below.

[CODE]
Array
(
[0] => Array
(
[role] => member
[username] => aaa
[datetime] => 1433566756
)

[1] => Array
(
[role] => member
[username] => aaa
[datetime] => 1433566755
)

[2] => Array
(
[role] => staff
[username] => ccc
[datetime] => 1433566754
)

[3] => Array
(
[role] => staff
[username] => ccc
[datetime] => 1433566753
)

[4] => Array
(
[role] => staff
[username] => ccc
[datetime] => 1433566752
)

[5] => Array
(
[role] => member
[username] => aaa
[datetime] => 1433566751
)

)

[/CODE]

please help…

thanks in advance

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