PHP, admins usersystem

You should definitely use IN clause in your SQL to do this. Selecting everything from the table in order to determine in PHP if it contains the user names you're looking for makes no sense and is very wasteful. Can you imagine what would happen if you had a table of 1 million users and you needed to see if two of them were on that list? You would be asking your DBMS to return 1 million rows to PHP so that you can search through each of those names and then determine whether or not any of them are the ones you're looking for. You're asking your DBMS to do a lot of work (send over all the rows in the table), and you're also asking PHP to do a lot of work (store all those rows in memory and compute a match), unnecessarily.

There is a much more efficient and faster solution depending on what you want.

First, if you only need to know that all of those users exist in the table then use SELECT COUNT(username) instead and your database will return a single row with a value for how many rows were found in the table. That way you have an all or nothing approach (if that's what you're looking for). Either there were 3 rows found in the table and 3 elements in the array or there weren't. This also utilizes your table indexes (which you should have properly indexed) and means faster results.

$admins = array("username","username2","username3");
// Make sure you properly escape your data before you put in your SQL
$list = array_map('mysql_real_escape_string', $admins);
// You're going to need to quote the strings as well before they work in your SQL
foreach ($list as $k => $v) $list[$k] = "'$v'";
$list = implode(',', $list);
$users = "SELECT COUNT(username) FROM usrsys WHERE username IN($list)";
$query_users = mysql_query($users);
if (!$query_users) {
    echo "Huston we have a problem! " . mysql_error(); // Basic error handling (DEBUG ONLY)
    exit;
}
if (false === $result = mysql_fetch_row($query_users)) {
    echo "Huston we have a problme! " . mysql_error(); // Basic error handling (DEBUG ONLY)
}
if ($result[0] == count($admins)) {
    echo "All admins found! We have {$result[0]} admins in the table... Mission complete. Returning to base, over...";
}

If you actually do want all the data then remove the COUNT from the SQL and you will simply get all the rows for those users (if any are found).

$admins = array("username","username2","username3");
// Make sure you properly escape your data before you put in your SQL
$list = array_map('mysql_real_escape_string', $admins);
// You're going to need to quote the strings as well before they work in your SQL
foreach ($list as $k => $v) $list[$k] = "'$v'";
$list = implode(',', $list);
$users = "SELECT username FROM usrsys WHERE username IN($list)";
$query_users = mysql_query($users);
if (!$query_users) {
    echo "Huston we have a problem! " . mysql_error(); // Basic error handling (DEBUG ONLY)
    exit;
}
// Loop over the result set
while ($result = mysql_fetch_assoc($query_users)) {
    echo "User name found: {$result['username']}\n";
}

However, I really urge you to reconsider using the old ext/mysql API to interface with your MySQL database in PHP since it is deprecated and has been discouraged from use for quite some time. I would really urge you to start using the new alternative APIs such as PDO or MySQLi and see the guide in the manual for help with choosing an API.

In PDO, for example this process would be quite simple with prepared statements and parameterized queries as you don't have to worry about all this escaping.

There's an example in the PDOStatement::Execute page (Example #5) that shows you just how to do use the IN clause that way with prepared statements... You can then reuse this statement in other places in your code and it offers a performance benefit as well as making it harder for you to inadvertently expose yourself to SQL injection vulnerabilities.

// Connect to your database
$pdo = new PDO("mysql:dbname=mydb;host=127.0.0.1", $username, $password);

// List of admins we want to find in the table
$admins = array("username","username2","username3");

// Create the place holders for your paratmers
$place_holders = implode(',', array_fill(0, count($admins), '?'));

// Create the prepared statement
$sth = $dbh->prepare("SELECT username FROM usrsys WHERE username IN ($place_holders)");

// Execute the statement
$sth->execute($admins);

// Iterate over the result set
foreach ($sth->fetchAll(PDO::FETCH_ASSOC) as $row) {
    echo "We found the user name: {$row['username']}!\n";
}

Your PHP code even looks so much better with PDO :)

Just include admins.php file and use the next construction in your loop:

while ($row = mysql_fetch_array($users)) {
if (in_array($users[0], $admins))
    echo $users[0];
}

Try this:

<?php
# include admins.php file that holds the admins array
include "admins.php";

# join all values in the admins array using "," as a separator (to use them in the sql statement)
$admins = join(",", $admins);

# execute the query
$result = mysql_query("
    SELECT username
    FROM usrsys
    WHERE username IN ($admins)
");

if ($result) {
    while ($row = mysql_fetch_array($result)) {
        echo $row["username"] . "<br>";
    }
}
?>

If your looking for syntax to pull in only the users from your $admins array then you could use something like:

$users="SELECT username FROM usrsys WHERE username IN ('".join("','",$admins)."')";

Where the php function JOIN will print username,username2,username3. Your resulting MySQL statement will look like:

SELECT username FROM usrsys WHERE username IN ('username','username2','username3')

Alternatively, if your looking to iterate through your $query_vars array and separate your admins from non-admins then you could use something like:

<?php
while($row = mysql_fetch_assoc($query_users)){
    if(in_array($row['username'],$admins)){
        //do admin stuff here
    }else{
        //do NON-admin stuff here
    }
}?>