通过ajax更新用户个人资料

hi guys im trying to update user details via ajax but its not updating my users table and just stop executing when possessing... appear here is my code please help if possible

edit_profile.php

<?php
if(isset($_POST["la"])){
$firstname = strip_tags(preg_replace('#[^a-z0-9]#i', '', $_POST['fi']));
    $lastname = strip_tags(preg_replace('#[^a-z]#', '', $_POST['la']));
    $age = strip_tags(preg_replace('#[0-9]#', '', $_POST['ag']));
    $gender = strip_tags(preg_replace('#[^a-z]#', '', $_POST['g']));

  // FORM DATA ERROR HANDLING
    if($gender == "" || $firsname == "" || $lastname == "" || $age == ""){
        echo "some account info is empty.";
        exit();
        }else{
      $sql = "UPDATE users SET gender='$gender', age='$age', lastname='$lastname, firstname='$firstname' WHERE username='$log_username' LIMIT 1";


     $query = mysqli_query($db_conx, $sql); 
echo "account_success";
        exit();

    }

    }
?>

ajax and js

<script type="text/javascript" language="javascript">
function _(x){
    return document.getElementById(x);
}
function toggleElement(x){
    var x = _(x);
    if(x.style.display == 'block'){
        x.style.display = 'none';
    }else{
        x.style.display = 'block';
    }
}

function restrict(elem){
    var tf = _(elem);
    var rx = new RegExp;
    if(elem == "firstname"){
        rx = /[^a-z0-9]/gi;
    } else if(elem == "lastname"){
        rx = /[^a-z0-9]/gi;
        } 
    tf.value = tf.value.replace(rx, "");
}
function emptyElement(x){
    _(x).innerHTML = "";
}
function ajaxObj( meth, url ) {
    var x = new XMLHttpRequest();
    x.open( meth, url, true );
    x.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    return x;
}
function ajaxReturn(x){
    if(x.readyState == 4 && x.status == 200){
        return true;    
    }
}
function update_profile() {
    var fi = _("firstname").value;
    var la = _("lastname").value;
    var g = _("gender").value;
    var ag = _("age").value;
    var status2 = _("status_profile");
    if(fi == "" || la == "" || ag == "" || g == ""){
        status2.innerHTML = "Fill out all of the form data";
        status2.style.color = "red";
    }else {
        _("profilebtn").style.display = "none";
        status2.innerHTML = 'processing ...';
        var ajax = ajaxObj("POST", "edit_profile.php");
        ajax.onreadystatechange = function() {
            if(ajaxReturn(ajax) == true) {
                if(ajax.responseText != "account_success"){

                    _("profilebtn").style.display = "block";
                } 
            }
        }
        ajax.send("&fi="+fi+"&la="+la+"&g="+g+"&ag="+ag);   
    }
}
</script>

the html

 <div class="row profile-classic">
<div class="col-md-12 m-t-20">
<form name="updateform" id="updateform" onsubmit="return false;">
<div id="profile_form" class="panel">
 <div class="panel-title line">
 <div class="caption"><i class="fa fa-phone c-gray m-r-10"></i>ACCOUNT</div>
 </div>
 <div class="panel-body">

 <div class="row">
<div class="control-label c-gray col-md-3">Username:</div>
<div class="col-md-6">
 <input type="text" class="form-control"  value="<?php echo $uname; ?>" disabled="disabled">
   </div>
   </div><br />
 <div class="row">
 <div class="control-label c-gray col-md-3">Firstname: *</div>
<div class="col-md-6">
<input type="text" onfocus="emptyElement('status_profile')" class="form-control" id="firstname" contenteditable="true" value="<?php echo $firstname; ?>">
 </div>
  </div><br />
 <div class="row">
  <div class="control-label c-gray col-md-3">Lastname: *</div> 
<div class="col-md-6">
 <input type="text" onfocus="emptyElement('status_profile')" class="form-control" id="lastname" placeholder="your lastname" value="<?php echo $lastname; ?>" >
 </div>
</div><br />

 <div class="row">
 <div class="control-label c-gray col-md-3">Age: *</div> 
<div class="col-md-6">
 <input type="text" onfocus="emptyElement('status_profile')" class="form-control" id="age" value="<?php echo $age; ?>">
</div>
 </div><br />
<div class="row">
 <div class="col-md-6">
<select class="form-control"  onfocus="emptyElement('status_profile')" id="gender" class="form-control">
<option value="<?php echo $gender; ?>"><?php echo $sex; ?></option>
<option value="m">Male</option>
 <option value="f">Female</option>

 </select>
 </div>
<button id="profilebtn" onclick="update_profile()" class="btn btn-sm btn-default"><i class="fa fa-floppy-o"></i> Save</button><span id="status_profile"></span>


</div><br />
  </div>
 </div></form>

</div></div>

You can use mysqli_error($db_conx) to determine what the error is... just have your javascript display it nicely for you.

But also - the variable in your WHERE clause ( $log_username ) is undefined.