Help with form validation – AJAX PHP JAVASCRIPT

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I am trying to validate a user name but I am running in to problems. I wrote a small PHP that checks is a user exists:

^<?php

^{require_once('Connections/myConnection.php');}

 

^{$user = $_GET['user'];}

 

^{$sql = sprintf("SELECT * FROM login WHERE username = '%s'", $user );}

^{$check_user = $myConnection->SelectLimit($sql) or die($myConnection->ErrorMsg() . "<br />" .$sql ."<br />");}

 

^{if($check_user->RecordCount() == 0)}

  ^{echo "1";}

^else

  ^{echo "0";}

^?>

 

Then I have the java script:

^{function checkUser(user)}

^{

^{xmlHttp=GetXmlHttpObject()}

^{if (xmlHttp==null)}

^{

^{alert ("Browser does not support HTTP Request")}

^return

^}

^{var url="check_user.php?user=" + user;}

 

^{xmlHttp.onreadystatechange=stateChanged}

^{xmlHttp.open("GET",url,true)}

^{xmlHttp.send(null)}

^}

 

^{function stateChanged()}

^{

^{if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")}

^{

^{var res = xmlHttp.responseText;}

^{else if(res == 1)}

^{

^{userCheckR = true;}

^{document.signup_form.checkUserVal.value = "true";}

^}

^else

^{

^{userCheckR = false;}

^{document.signup_form.checkUserVal.value = "false";}

^}

^}

^}

 

^{function GetXmlHttpObject()}

^{

^{var xmlHttp=null;}

^try

^{

^{// Firefox, Opera 8.0+, Safari}

^{xmlHttp=new XMLHttpRequest();}

^}

^{catch (e)}

^{

^{//Internet Explorer}

^try

  ^{

  ^{xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");}

  ^}

^{catch (e)}

^{

  ^{xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");}

^}

^}

^{return xmlHttp;}

^}

 

 

And in the form validation I do the following:

 

 

^onClick="

^{var valForm = false;}

^{checkUser(document.signup_form.username.value);}

                   

^{if(document.signup_form.checkUserVal.value == 'true'){}

    ^{valForm = true;}

^}

^{else {}

    ^{alert('The user name is already used by another account. Please try a different user name.');}

    ^{valForm = false;}

^}...

 

 

I think that the problem is that the respond from ajax is registered only after the I check the values. Is there a better way? Anything would be great.