我尝试将AJAX变量混合到PHP时出现问题

So I'm trying to make the simplest of communications between AJAX and PHP, and send a single variable to my PHP, where it will be stored, but nothing seems to work u_u

This here is my AJAX code

$(document).ready(function() {

 $("#ch1").click(function(){

  document.getElementById('stf').style.visibility="visible";
  document.getElementById('ch2').style.visibility="visible";
  document.getElementById('reto1').src="slides/reto1_completo.png";
  var counter = 1;
  var longform = $("input:text").serialize();
  $.ajax({
   type: GET,
   url: 'counter.php',
   data:   longform + "counter=<?php echo $counter; ?>",
  })
 })
})

And this one here is my PHP

session_start();

$db = mysql_connect("localhost","db","pass");
if(!$db) die("Error");
mysql_select_db("db",$db);

$counter = $_GET['counter'];

$insert = mysql_query("UPDATE usuarios SET retos='$counter' WHERE email = '$correo'") or die(mysql_error());

So this is it, I'm sure it's the simplest of mistakes, but I just can't figure it out. Thanks in advance

You're building your data string wrong. longform will look something like:

foo=bar&baz=qux

which you then tack on the coutner value directly:

foo=bar&baz=quxcounter=$counter

You need a & separator:

   data:   longform + "&counter=<?php echo $counter; ?>",
                       ^---here

so counter will be its own key, not part of the baz value that's at the end of longform.

What about this?

$.get("url.php?"+longform+"&counter=1");

Edit: Or

longform.counter = 1;
$.get("url.php",longform);

instead of GET you must write "GET", with quotes.
moreoever, your <?php echo $counter; ?> doesn't write anything because the counter insn't defined inside that php code, so you must replace that by another method for finding count (or use for instance SET retos=retos+1 in the mysql, it you want that).

Selecting some parts of your code and correcting it I achieved this, to which I suggest you take a look, because it runs well (and then you add the other parts):

<html>

<head>
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script type="text/javascript">
$(document).ready(function() {
    $("#ch1").click(function(){
        alert("hey!");
        $.ajax({
            type: "GET",
            url: './counter.php',
            data: "counter=<?php echo '...'; ?>",
            success: function(data, text_status, jqXHR){
                alert(data);
            },
            error: function(XML_http_request, text_status, error_thrown){
                alert(error_thrown);
            }
        })
    });
})
</script>
</head>

<body>
<div id="ch1">hello world!</div>
</body>

</html>

in the other php file:

<?php
# connect to the database...
# OK...

$counter = $_GET['counter'];

# change database's info...
# OK...

echo $counter;  #for debug purposes, to verify that in this code the php is receiving the string passed by the html.
?>