Pulling specific random values from php array - php

Question

On the page I'm creating there is graphic representation of pigeon-holes with five compartments for new comments. If there is new unread comment it should show up as graphic in one random compartment. But it won't happen if all of the 5 are
already occupied. So if someone writes new comment I like the code to check for if there is already five of them taken, and if not, than to randomly occupied one of the remaining ones. So "new_c" stands for number of
unread (new) comments, and c1-c5 stands for compartments. Value 0 means empty compartment for each "c". I was trying to make code that would first separate new_c from rest of the array after knowing the number is smaller than 5,
so I'm only working with 5 compartments. And than to determine which one is/are empty. And then randomly choose one empty to occupy. It's not really working as it is I probably am not using that array_keys properly as c2 is changed to some other value than 0 but still is being echoed, there is probably also a better/more efficient way to achieve that. I will really appreciate some input.
<?php
$tucQuery = "SELECT new_c,c1,c2,c3,c4,c5 FROM users WHERE id = '{$author_id}' LIMIT 1";
$result_tuc = mysqli_query($connection, $tucQuery);
$tucArray = mysqli_fetch_assoc($result_tuc);
mysqli_free_result($result_tuc);
$new_c = $tucArray[0];
if($new_c < 5){
$new_array = array_keys((array_slice($tucArray,1)), 0);
$rand_zero = array_rand($new_array, 1);
echo $rand_zero + 1;
}
?>
Bellow code works but it doesn't seem to be efficient and there is probably a better way.
if($new_c < 5){
$empties = array();
if($tucArray['c1'] == 0){
$empties[] = 1;
}
if($tucArray['c2'] == 0){
$empties[] = 2;
}
if($tucArray['c3'] == 0){
$empties[] = 3;
}
if($tucArray['c4'] == 0){
$empties[] = 4;
}
if($tucArray['c5'] == 0){
$empties[] = 5;
}
print_r($empties);
$rand_zero = array_rand((array_flip($empties)), 1);
echo $rand_zero;
}