Merge arrays in PHP based on keys

I have this array:

Array
(
    [0] => Array
        (
            [id_decoder] => 1
            [id_cliente] => 1
            [id_gruppo] => 2
            [decoder] => decoder1
            [username] => decoder1
            [password] => decoder1
            [note] => decoder1
            [nome] => primo
            [cognome] => primini
            [nome_gruppo] => mascia
        )

    [1] => Array
        (
            [id_decoder] => 2
            [id_cliente] => 2
            [id_gruppo] => 1
            [decoder] => decoder2
            [username] => decoder2
            [password] => decoder2
            [note] => decoder2
            [nome] => secondo
            [cognome] => secondini
            [nome_gruppo] => miei
        )

    [2] => Array
        (
            [id_decoder] => 3
            [id_cliente] => 3
            [id_gruppo] => 2
            [decoder] => decoder3
            [username] => decoder3
            [password] => decoder3
            [note] => decoder3
            [nome] => Terzo
            [cognome] => Terzini
            [nome_gruppo] => mascia
        )

)

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And this array:

Array
(
    [id_decoder] => 1
    [mesi_pagati] => 2
)
Array
(
    [id_decoder] => 2
    [mesi_pagati] => 1
)

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I would like to merge the "mesi_pagati" only to its relative id_decoder.

How can I obtain that in php?

Maybe part of the problem is that the second array is made by two arrays.

I obtain this with this code:

$row = $result->fetch_array(MYSQLI_ASSOC);

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But I can change it to numeric or to anything else if helps obtain the final result of a single array with all needed data.