mysql - Php Ensuring a unique username -

i have register page , want ensure usernames unique. have tried solutions on here , nothing working me.

this error i'm getting:

warning:  mysqli_query() expects @ least 2 parameters, 1 given  in c:\xampp\htdocs\project\projectregistered.php on line 16 query empty 

here php:

 <?php  $con=mysqli_connect("localhost","root","","test");  // check connection  if (mysqli_connect_errno()) {      echo "failed connect mysql: " . mysqli_connect_error();  }  $username = $_post['username'];    echo $username;  $query = mysqli_query("select * projectuser username='$username'");  $result1 = mysql_query($query) or die (mysql_error().$query);     $count = mysql_num_rows($result1);  if ($count>0) { echo 'sorry! username exists!'; } else {     $surname = $_post['surname'];     $sql = "insert projectuser (firstname, surname, username, password, location)             values             ('$_post[firstname]', '".mysql_real_escape_string($surname)."', '$_post[username]', '$_post[password]', '$_post[location]')"; }  if (!mysqli_query($con,$sql)) {     die('error: ' . mysqli_error($con)); } echo "1 record added ".$sql;  mysqli_close($con);  ?> 

as you're using procedural approach, need pass connection handle first argument. should

mysqli_query($con, "select * projectuser username='$username'"); 

additionally, make username column in db table unique.

update:
realised you're using both mysql_ mysqli_ functions. better stick second one. code be:

$query = "select * projectuser username='$username'"; $result = mysqli_query($con, $query); $count = mysqli_num_rows($result); 

but mentioned above, if set username column unique won't need check @ all. use mysqli_insert_id after insert check whether record has been inserted or not. if hasn't been inserted, unique constraint had fired off. way more efficient selecting first check whether username exists.