<?php
$valid = array(1,2,3);
$post = array('1','f','3', '3');
if(is_array($post)){
    $post = array_unique($post);
    foreach($post as $key => $value){
        if(in_array($value, $valid)){
            $class = true;
        }else{
            unset($post[$key]);
        }
    }
}
if(isset($class)){
    echo 'class="class' . implode(' class', $post) . '"';
}