<?php $arr = array_map(function($v) { return (array) $v; }, $arr);?>

Было

<?php
Array
(
    [0] => stdClass Object
        (
            [attr_1] => 318
            [count] => 2
        )

    [1] => stdClass Object
        (
            [attr_1] => 319
            [count] => 1
        )

    [2] => stdClass Object
        (
            [attr_1] => 320
            [count] => 4
        )

)
?>

Стало

<?php
Array
(
    [0] => Array
        (
            [attr_1] => 318
            [count] => 2
        )

    [1] => Array
        (
            [attr_1] => 319
            [count] => 1
        )

    [2] => Array
        (
            [attr_1] => 320
            [count] => 4
        )
)
?>

Обратный процесс

<?php $arr = array_map(function($v){ return (object) $v; }, $arr); ?>