Migrating MySQL site to Oracle Linux and new PHP

[Jason Kinninger]

I originally had a site that read a MySQL database running on SUSE Linux running PHP 5.3 and MySQL 5.5. I am attempting to migrate the site to Oracle Linux 76. with PHP 7.2.16 with MySQL 8.0.15. I have all the data migrated to the new database I just can't get the webpage to show the table content.

My index.php looks like this -
<?php
include 'db/config.php';
include 'db/opendb.php';

$output = "";

// $sql = "select url from prime_cache_list where enabled = 1";
$sql = "select url from branch_cache.prime_cache_list where enabled = 1 order by dt_updated desc";
$res = mysqli_query($sql) or trigger_error(mysqli_error()." in ".$sql);

while($row = mysqli_fetch_assoc($res)) {
$output .= $row['url'] . "\n";
}

header("Content-Type:text/plain");
echo $output;
?>

When I try to access the site I am getting this error - PHP Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, null given in /var/www/html.branchcache/index.php on line 11

I was reading where in version 8 I should use mysqli instead of the older mysql_ commands. Not sure why it is getting null data when I can see the data within the database. Should I use something other than mysqli_fetch_assoc to get my data?