PHP search an array value

Hey freakers,

I am stumbling upon a problem I dont know how to solve, so here it is. Its something of a luxery problem, even if I cant fix it I dont mind. However, I would like to know if its possible.

What I would like is this. I have this array:

Array
(
    [0] => Array
        (
            [comment_id] => 1
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 0
            [time] => March 19th, 5:24AM
            [content] => Some text
        )

    [1] => Array
        (
            [comment_id] => 2
            [user] => Array
                (
                    [id] => 2
                    [name] => Jason Biondo
                    [avatar] => /assets/img/user_images/2_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:24AM
            [content] => Some text
        )

    [2] => Array
        (
            [comment_id] => 5
            [user] => Array
                (
                    [id] => 3
                    [name] => Rudolph
                    [avatar] => /assets/img/user_images/3_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:45AM
            [content] => Some text
        )

    [3] => Array
        (
            [comment_id] => 6
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:45AM
            [content] => Some text
        )

    [4] => Array
        (
            [comment_id] => 4
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 2
            [time] => March 19th, 5:25AM
            [content] => Some text
        )

    [5] => Array
        (
            [comment_id] => 3
            [user] => Array
                (
                    [id] => 3
                    [name] => Rudolph
                    [avatar] => /assets/img/user_images/3_avi.jpg
                )

            [parent_id] => 0
            [time] => March 19th, 5:25AM
            [content] => Some text
        )

    [6] => Array
        (
            [comment_id] => 7
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 0
            [time] => March 19th, 7:47AM
            [content] => Some text
        )

)

This is an array of comments on a blog. Regardless, I would like to focus this area in specific:

Array
(
    [0] => Array
        (
            [comment_id] => 1
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 0
            [time] => March 19th, 5:24AM
            [content] => Some text
        )

    [1] => Array
        (
            [comment_id] => 2
            [user] => Array
                (
                    [id] => 2
                    [name] => Jason Biondo
                    [avatar] => /assets/img/user_images/2_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:24AM
            [content] => Some text
        )

    [2] => Array
        (
            [comment_id] => 5
            [user] => Array
                (
                    [id] => 3
                    [name] => Rudolph
                    [avatar] => /assets/img/user_images/3_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:45AM
            [content] => Some text
        )

    [3] => Array
        (
            [comment_id] => 6
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 1
            [time] => March 19th, 5:45AM
            [content] => Some text
        )

    [4] => Array
        (
            [comment_id] => 4
            [user] => Array
                (
                    [id] => 1
                    [name] => Davey Kulter
                    [avatar] => /assets/img/user_images/1_avi.jpg
                )

            [parent_id] => 2
            [time] => March 19th, 5:25AM
            [content] => Some text
        )
)

as you can see from parent_id,  the first is a comment and the rest are replies to that comment (I only allow one level of replies, so no reply to a reply).

However, if I loop through this array in a foreach() loop and I am at array key [4], how do I find out what the comment_id of the previous array key where parent_id = 0. SO, what I would like is a function that would, in this case, return id = 1.

If I dont make sense, I am sorry. Its quite weird xD

Thanks regardless