I am getting mysqli error message but it is still returning message

Question

I am using AJAX to call the below method on my register page to validate the form, I have copied in the lines where I am experiencing trouble. I get this message Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in /home/waexchan/public_html/signup.php on line 15 but on the website the message Username is OK is shown so I am confused. Line 15 is the line beginning with $uname_check

?><?php
if(isset($_POST["usernamecheck"])){
include_once("db_conx.php");
$username = preg_replace('#[^a-z0-9]#i', '', $_POST['usernamecheck']);
$sql = "SELECT username from User WHERE username'$username' LIMIT 1";
$query = mysqli_query($db_conx, $sql); 
$uname_check = mysqli_num_rows($query);
if (strlen($username) < 3 || strlen($username) > 16) {
    echo '<strong style="color:#F00;">3 - 16 characters please</strong>';
    exit();
}
if (is_numeric($username[0])) {
    echo '<strong style="color:#F00;">Usernames must begin with a letter</strong>';
    exit();
}
if ($uname_check < 1) {
    echo '<strong style="color:#009900;">' . $username . ' is OK</strong>';
    exit();
} else {
    echo '<strong style="color:#F00;">' . $username . ' is taken</strong>';
    exit();
}

}

Answer 1

Your sql query seems wrong. Change your query accordingly:

Eg.

 $sql = "SELECT username from User WHERE username='$username' LIMIT 1";