Its just an idea
function filter($var)
{
return preg_replace('/[^a-zA-Z0-9]/', '', $var);//you can arrange replacements as per your requirement
}
$getstr = filter($_POST['string']);
if(strcmp($getstr, $_POST['string']) === 0 ){
//statement here ----
return true;
}else{
header('HTTP/1.1 400 Bad Request');
}