Q:

If password in textbox doesn't match databases', stop FORM from submitting

In the update password form, if the current password is not same as what is there in the database how should I stop the form from submitting? 
Presently, I have a form wherein using AJAX I am checking if the password in the textbox is same as that in the database and if not, it displays a message that password is incorrect but when I submit the form with a new password, the password gets updated. 
I want that if password in textbox is not the same as is present in the database, execution should stop until correct password is not entered.
FORM

 <h5>Update Password</h5>
            </div>
            <div class="widget-content nopadding">
              <form class="form-horizontal" method="post" action="#" name="password_validate" id="password_validate" novalidate="novalidate">
                <div class="control-group">
                  <label class="control-label">Current Password</label>
                  <div class="controls">
                    <input type="password" name="current_pwd" id="current_pwd" />
                    <span id="chk_pwd"></span>
                  </div>
                </div>
                  <div class="control-group">
                  <label class="control-label">New Password</label>
                  <div class="controls">
                    <input type="password" name="new_pwd" id="new_pwd" />
                  </div>
                </div>
                <div class="control-group">
                  <label class="control-label">Confirm Password</label>
                  <div class="controls">
                    <input type="password" name="confirm_pwd" id="confirm_pwd" />
                    <span id="chk_confirm_pwd"></span>
                  </div>
                </div>
                <div class="form-actions">
                  <input type="submit" name="submit" value="Validate" class="btn btn-success">
                </div>
              </form>

JQUERY

 $(document).ready(function(){
            $('#new_pwd').click(function(){
                var current_pwd=$('#current_pwd').val();
                $.ajax({
                    type:'get',
                    url:'admin_profile_settings_back.php',
                    data:{current_pwd:current_pwd},
                    //dataType:"html",
                    success:function(resp){
                     $("#chk_pwd").html(resp).show().fadeOut(3000);
                     
                 }
                })
            })
        })

PHP Backend for AJAX

<?php
 session_start();
 include "../includes/config.php";
 
 
    $current_pwd=$_GET['current_pwd'];
    $aid=$_SESSION['aid'];
    
    $r=$mysqli->query("SELECT * FROM admin WHERE aid='$aid' ");
    $row=$r->fetch_assoc();
    
    if($current_pwd==$row['password'])
    {
        echo "Password Is Correct";
    }
    else
    {
        echo "Password Is Incorrect";
    }

A:

You should include a where clause to check for the current password.
You should hash + salt your password
This example does not have any input checks, you should always verify user input, ALWAYS.
You should use the PDO library and perform prepare statements, is more secure than mysqli, also you can mysqli_real_escape_string() at minimum

<?php
 session_start();
 include "../includes/config.php";

    $current_pwd=$_GET['current_pwd'];
    $aid=$_SESSION['aid'];
    $current_pwd = $mysqli->real_escape_string($current_pwd);

    $r=$mysqli->query("SELECT * FROM admin WHERE aid='$aid' and password='$current_pwd' ");
    $row=$r->fetch_assoc();

    if($current_pwd==$row['password'])
    {
        echo "Password Is Correct";
    }
    else
    {
        echo "Password Is Incorrect";
    }

