我在php连接错误？

I have the following, which is not outputting "OK" in my browser. I assume the concatenation is wrong. I would be grateful for any help. Cheers.

<?php
$x = array("submit","submit1");

foreach ($x as $value){
if ((isset($_POST['$value . "_" . " x"']))){
echo "OK";
}
?>

I want the content of $_POST to be submit_x, submit1_x etc

As requested, print_r($POST) gives in the browser:

OK
Notice: Undefined index: submit1_x in C:\xampp\htdocs\lotto\test1.php on line 18

You made it more complicated than it needed to be:

 $_POST[$value . "_x"]