Difficulty checking for existing user registration php/mysqli

Question

I am having issues denying the registration of existing users. please see code below, and further details below that.

<?php
//connect to sql
 $con=mysqli_connect("host","user","pass","db");
  if (mysqli_connect_errno())
   {
   echo "Failed to connect to MySQL: " . mysqli_connect_error();
   }

//pull user id from form input
$uid=$_POST[uid];

//make sure input is not blank
if($uid==""){echo 'Please enter a username.';exit();}

//make sure it is alphanumeric input only
if(!ctype_alnum($uid)){echo 'The username chosen must be alpha-numberic (A-z,0-9),   
without special characters. Please use the back button in your
browser to try again using only alpha-numeric input.';exit();}


//check if user exists
$query=mysqli_query($con, "SELECT * FROM user WHERE uid='$uid'");
if(!$query){die 'query failed to execute';}

if(mysqli_num_rows($query)!=0){echo 'user already exists. Use back button and enter a new username'; exit();
}

//insert record into DB
$sql= "INSERT INTO user (uid, addr, password, passwordlastchanged, email)
 VALUES
 ('$uid', '$_POST[addr]', '$_POST[pwd]','today', '$_POST[email]')";

 if (!mysqli_query($con,$sql))
   {
   die('Error: ' . mysqli_error($con));
   }
 echo "1 record added";

 mysqli_close($con);
 ?> 

So when I input through my form it works fine, its just that if I try to register an already existing username, it allows it, when I have setup the query to look for an existing entry and cancel if it finds it (or so i thought...).

can you see what I am doing wrong? again I want it to check for the user name and deny further action if it finds it..

I know this is probably lacking in security, I am unfortunately learning as I go so just getting things to work comes first :) Although security suggestions will be logged and considered at the right time.

I spent an enormous amount of time researching this and I feel like I've tried a lot of various query/ function combos.

Answer 1

Try

if(mysqli_num_rows($query)>0) 

instead of

 if(mysqli_num_rows($query)!=0)