使用Javascript将变量传递给PHP MySQL脚本

Question

I borrowed this code from another source. Now, I am attempting to modify it.

I need to pass the contents of $q to my php page and use this as a where clause in my SQL statement.

My Javascript:

<script>
  function subject(str) {
    if (str == "") {
      document.getElementById("subject").innerHTML = "";
      return;
    } else { 
      if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();
      }
      xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
          document.getElementById("subject").innerHTML = xmlhttp.responseText;
       }
     }
     xmlhttp.open("GET","form_get.php?q="+str,true);
     xmlhttp.send();
    }
  }
</script>

Inside the html select code I am using: onchange="subject(this.value)"

My PHP

$q = intval($_GET['subject']);
//if (!empty($_GET['q'])){
  //$q = $_GET['q'];
//}
include('../conn/conn.php');
$sql = "select DISTINCT grade FROM primary_skills where subject= $q ";

As you can see, I am passing the $q into my SQL statement. I understand that intval returns a number, but when I try other types, such as strval, it breaks the script. It als breaks the script when I tried the commented out section above.

When I change the php to: $q=$_GET["q"]; I get the error: form_get.php?q=Reading 500 (Internal Server Error). This tells me that $q is indeed pulling from the options list, but something else is going on...

Answer 1

$q = intval($_GET['subject']);

This looks wrong - should that not be $q = intval($_GET['q']); ?

Answer 2

the problem is with your php you suppose to get the q and not subject

$q = intval($_GET['q']);

include('../conn/conn.php');
$sql = "select DISTINCT grade FROM primary_skills where subject= $q ";