$server_maxclients = 5; // dynamic
$server_clientsonline = 1 // dynamic

<div class="progress-bar" role="progressbar" aria-valuenow="<?php echo $server_clientsonline; ?>" aria-valuemin="0" aria-valuemax="<?php echo $server_maxclients; ?>" style="background-color: #4ba84b;width: 20%;"></div>

$server_clientsonline = 4;
$server_maxclients = 50;

<?php echo ($server_clientsonline*100/$server_maxclients).'%';?>

<div
    class="progress-bar"
    role="progressbar"
    aria-valuenow="<?php echo $server_clientsonline; ?>"
    aria-valuemin="0"
    aria-valuemax="<?php echo $server_maxclients; ?>"
    style="background-color: #4ba84b;width: <?php echo ($server_clientsonline*100/$server_maxclients).'%';?>; height:30px;">
</div>