Understanding HTML form code - html

Question

i am bit confused about the code..This is login.php file..and action of the form is also in the same file.Can this happen? if,then same login form should open if user submit the form...i am making my website,where i want to use login/register form.
<form action="login.php" method="post" class="f-wrap-1">
<div class="req">
Not Registered?<br />
Forgot your Password?
</div>
<fieldset>
<h3>Member Login</h3>
<label for="firstname"><b>Username:</b>
<input id="username" name="username" type="text" class="f-name" autocomplete="on" tabindex="1" /><br />
</label>
<label for="password"><b>Password:</b>
<input id="password" name="password" type="password" class="f-name" autocomplete="off" tabindex="2" /><br />
</label>
<label for="code"><b>Security Code:</b>
<input id="code" name="code" type="text" class="f-name" autocomplete="off" tabindex="3" /><br />
</label>
<label for="code2"><b> </b>
<img src="image.php?" /><br />
</label>
<div class="f-submit-wrap">
<input type="submit" value="Submit" class="f-submit" tabindex="4" /><br />
</div>
</fieldset>
</form>

Answer 1

This is login.php file..and action of the form is also in the same file.Can this happen?
Yes
if,then same login form should open if user submit the form
Not necessarily. In a system like this, the form data will be processed by server side code. The logic will probably be something like:
If it is a GET request, send the browser the form.
Otherwise, if it is a POST request, then check the form data:
If it is valid login data, then: set a cookie to track the user and
tell the browser to get some other URL.
Otherwise, the login data is wrong: populate the form with an error message
and possibly default the values of the fields to the wrong data the user
entered, then send the form to the browser.
Populating the form with the invalid data doesn't make much sense in a login form like this one, but it more useful in (for example) a registration form.
Who wants to retype all their personal data again just because the username they wanted is not available or they missed a field?

Answer 2

It's possible to send form to same file but it's not in good style. You would have to check if post data is available and display proper view based on that.
Better way to do it is just change the action of the form to point to another file and in that file handle login logic.
<form action="file.php" method="post" class="f-wrap-1">