JQuery event.preventdefault and php

I have a question concerning event.preventdefault. I used it on my client side validation just to make sure a username and password were filled in. However when I pressed the submit button and passed the validation on to php I got nothing. I mean when I checked if the form was submitted and did an echo to check I got nothing. But when I did (!isset($_POST['whatever']) I got something. Same goes for check if the fields were empty. I got nothing when I did empty, but got something when I did !empty

Jquery/html code

  
    
    
    
    
    
    
  

  
    

      
      

        
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php code

    //include the connection to the cms database
    require_once($_SERVER['DOCUMENT_ROOT'].'/vcm/_cms/resources/database_connect.php');


    //check if the form was sent
    if(isset($_POST['loginToSystem'])){
        echo 'o';
        //create an array for errors
        $errors = array();

        //check if the username and password fields are empty
        //yes we did a check if jquery on the front but some people disable javascript
        if(empty($_POST['username']) || empty($_POST['password'])){
            echo 'empty';

        }else{
            echo 'ok';
        }
    }
    echo'p';

?>

So basically it echo's 'p' no matter what, but will only echo 'o' if isset is changed to !isset. It will echo empty when left as is, and ok when changed to !empty

Answer

as adeneo explained you must add name

        
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also you need && not ||

$("#cms-signin").submit(function(event){
        if($('#username').val() == "" && $('#password').val() == ""){
            event.preventDefault();
            $("#jsError").html('You must fill all fields.');
        }
    });