JavaScript验证后PHP函数无效[关闭]

I used validation for my login field with jQuery & Javascript, and it works. When there is no username or password the span text changes.

When the details are correct and I press login it just reloads the page again, instead of starting the session etc.

But if I enter a valid username with a wrong password, PHP kicks in and displays incorrect password or if I enter an invalid username, it says username not found.

This is such a weird problem. I have a feeling it's due to the fact I am returning a flag to check sending, after the Javascript validation:

$(document).ready(function() { 
    $('#loginf').submit(function(){ 
        var flag = true

        //stuff to validate each field 
        //ie: for each field i have:

       if(//empty field) { 
        //stuff flag= false; 
       } 
       return flag; 
   }); 

});