deny submitting form if the username exist on the mysql db
<p>i just pluged a <code>JQuery username check</code> which worked fine , the problem is that my form still submitting even the username exist on my <code>Mysq Database</code>, how can i configure it to deny submitting to my <code>server side .php file</code> if it exists ?</p>

<p>here is my Jquery plugin javascript code </p>

<pre><code>  $(document).ready(function() {


        //the min chars for checkusername
        var min_chars = 3;

        //result texts
        var characters_error = 'Minimum amount of chars is 3';
        var checking_html = '&lt;img src="images/checkusername.gif" /&gt; Checking...';

        //when button is clicked
        $('#check_checkusername_availability').click(function(){
            //run the character number check
            if($('#checkusername').val().length &lt; min_chars){
                //if it's bellow the minimum show characters_error text
                $('#checkusername_availability_result').html(characters_error);
            }else{          
                //else show the cheking_text and run the function to check
                $('#checkusername_availability_result').html(checking_html);
                check_availability();
            }
        });


  });

//function to check checkusername availability  
function check_availability(){

        //get the checkusername
        var checkusername = $('#checkusername').val();

        //use ajax to run the check
        $.post("frontend/functions/f_checkuser.php", { checkusername: checkusername },
            function(result){       
                //if the result is 1
                if(result == 1){
                    //show that the checkusername is available
                    $('#checkusername_availability_result').html('&lt;span class="is_available"&gt;&lt;b&gt;' +checkusername + '&lt;/b&gt; is Available&lt;/span&gt;');
                }else{
                    //show that the checkusername is NOT available
                    $('#checkusername_availability_result').html('&lt;span class="is_not_available"&gt;&lt;b&gt;' +checkusername + '&lt;/b&gt; is not Available&lt;/span&gt;');
                }
        });

} 
</code></pre>

<p>here is my html field</p>

<pre><code> &lt;table border="0" &gt;
          &lt;tr&gt;
            &lt;td valign="middle"&gt;&lt;input class="input_field_12em required userUserName" name="userUserName" id="checkusername"&gt;&lt;/td&gt;
            &lt;td valign="middle"&gt;&lt;input type='button' id='check_checkusername_availability' value='Check Availability'&gt;&lt;/td&gt;
            &lt;td&gt;&lt;div id='checkusername_availability_result'&gt;&lt;/div&gt;&lt;/td&gt;
          &lt;/tr&gt;
        &lt;/table&gt;
</code></pre>
