comment puis-je ajouter une condition qui va afficher une erreur si le nom d'utilisateur déjà pris ou existe

Question

Comment puis-je ajouter une condition qui affichera une erreur si le nom d'utilisateur déjà pris ou existe?

Ceci est mon code:

<?php 
    include '../connect.php'; 
    ?> 

    <?php 
    if(isset($_POST['register'])){ 

    $username = $_POST['username']; 
    $password = $_POST['password']; 
    $password2 = $_POST['password2']; 
    $email = $_POST['email']; 
    $fname = $_POST['fname']; 
    $lname = $_POST['lname']; 
    $bday = $_POST['bday']; 
    $contact = $_POST['contact']; 
    $address = $_POST['address']; 
    $licenseno = $_POST['licenseno']; 
    $gender = $_POST['gender']; 
    $picture = $_POST['picture']; 
    $avail = $_POST['availability']; 

    if(empty($username)||empty($email)||empty($password)||empty($password2)||empty($fname)||empty($lname)||empty($bday)||empty($contact)||empty($address)||empty($licenseno)||empty($gender)){ 
     $msg="*PLEASE FILL IN REQUIRED FIELDS*"; 
     header('Location:RegisterT.php?msg='.$msg.''); 
    } 
    else{ 

     if($password != $password2){ 
      $msg="Password Mismatch!"; 
      header('Location:registerT.php?msg='.$msg.''); 
      } 
     else{ 

      mysql_query("INSERT INTO therapist (`id`, `username`, `email`, `password`, `fname`, `lname`, `bday`, `contact`, `address`, `licenseno`, `gender`, `picture`, `about`, `availability`)VALUES 
      ('', '$username','$email','$password','$fname','$lname','$bday','$contact','$address','$licenseno','$gender','default.jpg','Currently no information to show','Available')"); 
      $msg="Registration Successful"; 
      header('Location:LoginT.php?msg='.$msg.''); 

     } 
    } 


} 

?> 

Answer 1

if($password != $password2) { 
     $msg="Password Mismatch!"; 
     header('Location:registerT.php?msg='.$msg.''); 
} 
else { 
    //Check for username in database. If exists, show error. 
    $sql = "SELECT username FROM therapist WHERE username = " . $username; 
    $result = mysqli_query($conn, $sql); 

    if (mysqli_num_rows($result) > 0) { 
     $msg="Username already exist"; 
     header('Location:LoginT.php?msg='.$msg.''); 
    } 

    mysql_query("INSERT INTO therapist (`id`, `username`, `email`, `password`, `fname`, `lname`, `bday`, `contact`, `address`, `licenseno`, `gender`, `picture`, `about`, `availability`)VALUES 
    ('', '$username','$email','$password','$fname','$lname','$bday','$contact','$address','$licenseno','$gender','default.jpg','Currently no information to show','Available')"); 
    $msg="Registration Successful"; 
    header('Location:LoginT.php?msg='.$msg.''); 

}