How to know if I am in the last iteration of a foreach

You can use the php array function end () . The function, as you can read in the link, establishes the pointer in the last element of the array. If you evaluate it with a conditional, you can achieve what you propose in a fairly clean and efficient way;

$id = $_POST['empId'];
$query = "UPDATE userInfo SET ";
$keys = array_keys($_POST);
foreach ($_POST as $key => $value) {
    if ($key === end($keys)) {
        $query .= "$key = '$value'";
    } else {
        $query .= "$key = '$value', ";
    }
}
$query .= " WHERE id = '$id'";

You can achieve what you want with the implode function, instead of concatenating to the string adds elements to an array and finally you do implode

$id = $_POST['empId'];
$query = "UPDATE userInfo SET ";
$queryParams = array();
foreach ($array as $key => $value) {
    $queryParams[] = "$key = '$value'";
}
$params = implode(', ', $queryParams);
$query = $query . $params . " WHERE id = '$id'";

As a side note, it seems that you are exposed to an attack by SQL Injection , in this post you speak in an extended manner on how to avoid it: How to avoid SQL injection in PHP?

It's simple, you should only check the keys of the arrangement and then check if you are actually in the last

<?php

//Leo cuantos elementos hay en el array
$cont = count($_POST);
//Consulto las keys del array
$keys = array_keys($_POST);
//Busco la ultima
$ultima_key = $keys[$cont-1];

foreach ($_POST as $key => $value) {
    if($key == $ultima_key){
        echo "Esta es la ultima iteracion\n";
    }else{
        echo "Esta no es la ultima iteracion\n";
    }    
}

?>