Access denied or blank page with mysqli but not mysql - php

Question

I've looked through StackOverflow for an answer to this and have so far come up empty handed. I know some have had a similar issue, but so far none of the responses to the issues have worked.
So I work on two sites, both of which are on the same host with the same PHP Admin set up (different accounts and domains, sites are not affiliated). One of them uses MySqli perfectly, without issues, while the other either gives a database selection failure, localhost access denied with password as "NO" or a blank page when I attempt to replace the MySql with MySqli.
The deprecated code:
$host = "LOCALHOST";
$usr = "USER";
$pwd = "PASSWORD";
$dbname = "DATABASE";
$connection = #mysql_connect($host,$usr,$pwd);
if(!$connection){
die("No connection.");
}
mysql_select_db($dbname,$connection);
The MySqli I am attempting (identical to the site that MySqli works perfectly on):
$host = "LOCALHOST";
$usr = "USER";
$pwd = "PASSWORD";
$dbname = "DATABASE";
$connection = mysqli_connect($host,$usr,$pwd);
if (!$connection) {
die("No connection.");
}
$db_select = mysqli_select_db($connection, $dbname);
if (!$db_select) {
die("Database selection failed: " . mysqli_error($connection));
}

Answer 1

So the PHP error codes did not work at all and it had nothing to do with the local host or port, so I first checked to make sure mysqli_connect was on. Next, I ran a simple test where I echoed a random word before each mysqli command.
echo 'Passed.';
global $c_mysqli;
$conn = new mysqli($host, $usr, $pwd, $dbname);
echo '<br>Passed global mysqli.';
I found that my second pass was not working, and then proceeded to do an error message. An error showed up then. However, nothing I did fixed the issue... then I realized that one of my files, the header file, is the meat and potatoes and if it were to fail then everything else would fail too.
Sure enough, one of my lines of code contained a mysql connection and not mysqli. Long story short, double check and triple check to make sure that all
$statement = mysql_query("STATEMENT");
$query = mysql_fetch_array($statement);
Appear as
$statement = $conn->query("STATEMENT");
$query = mysqli_fetch_array($statement);