PHP MySQL INSERT Query & PHP seems to be run twice - php

Question

I want to make simple login server for android.
Login feature works well, but in registration has some problem.
My intent is :
Android Client Send User Info(id, username, password) to XAMPP Server by using HTTPPOST.
Server get User Info, and find repetition id or username from database(using select query). if repetition exists, response to client using echo.
if repetition not exists, make new user info to database using insert query. And response to client using echo.
If repetition exists, it works well. server response correctly.
but in case of no repetition, query's output is 1 then server response 'there is repetition'...
login.php is same code with register.php, but there is no insert query in login.php, and it works perfectly well.
so I think this problem caused by insert query.
here is my code:
<?php
$hostname_localhost ="localhost";
$database_localhost ="mydatabase";
$username_localhost ="root";
$password_localhost ="";
$id_localhost ="";
$localhost = mysql_connect($hostname_localhost,$username_localhost,$password_localhost, $id_localhost) or trigger_error(mysql_error(),E_USER_ERROR);
mysql_select_db($database_localhost, $localhost);
$username = $_POST['username'];
$password = $_POST['password'];
$id = $_POST['id'];
$query_search = "select * from tbl_user where username = '".$username."' OR id = '".$id."'";
$query_exec = mysql_query($query_search) or die(mysql_error());
$rows = mysql_num_rows($query_exec);
if($rows == 0) {
$query_register = "INSERT INTO tbl_user (id, username, password) VALUES ('$id', '$username', '$password')";
$result = mysql_query($query_register) or die(mysql_error());
if($result == TRUE){
echo "Register Success";
} else {
echo "Register Fail";
}
exit;
die;
} else {
echo "Registered Device or Username";
exit;
}
?>
login.php is almost same. only search query(select id and username and password same) and insert query is different(no insert in login.php).
server response is wrong but insert action is doing right. I can see new record in my database. so... it seems to be run twice.
client side has no problem because it also almost same with login code(And login works well with no insert query:)).

Answer 1

Update
mysql_num_rows return an int, you are checking if $rows is false instead of 0. Use ===.
What id are you referring to in your tbl_user and why would the user supply their ID? Are you sure that you are checking for the right id? It feels like you should be doing a separate query for the device id OR check for a device_id column in the table tbl_user.
Other things to look into
exit; and die; are the same so calling it twice makes no sense.
if ($rows == 0) is checking for (bool) false instead of the integer 0. Use === for checking for supplied type.
When using "" double-quotation marks you can supply the variables directly;
E.g. "...WHERE username = '$username'" instead of "... WHERE username = '".$username."'".
Look into using mysqli instead of mysql for more features and improved security; http://php.net/manual/en/book.mysqli.php