将数组替换为二维数组中的数字

I have two-dimensional array and I want replace second of two-dimensional array on random number of second array

Array
(
    [1] => Array
        (
            [0] => 1
            [1] => 3
            [2] => 5
            [3] => 500
            [4] => 600
            [5] => 700
        )

    [2] => Array
        (
            [0] => 2
            [1] => 4
            [2] => 6
        )

)

I want get

 Array (
     [1] => 5 (<- random from first array)     
     [2] => 6 (<- random from second array)    
 )

I tried to do:

foreach($variables as $key => $val) {
        $variables = str_replace($val, $val[array_rand($val)], $variables);
}

Why it doesnt work?

foreach($variables as $key => $val) {
        $variables[$key] = $val[array_rand($val)];
}

foreach ($variables as &$var) {
     $var = array_rand($variables[$var]);
}