user registration with salt in php

<?php
$con=mysqli_connect("localhost","girishgupta","dsv@2018","icengodb");
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$message='';
if(isset($_POST['register']))
{
$username = mysqli_real_escape_string($con, $_POST['username']);
$password = mysqli_real_escape_string($con, $_POST['password']);
$email = mysqli_real_escape_string($con, $_POST['email']);
$firstname = mysqli_real_escape_string($con, $_POST['firstname']);
$lastname = mysqli_real_escape_string($con, $_POST['lastname']);
$status=1;
$used=0;
 $date_added = date('Y-m-d H:i:s');
function generateSalt($max = 9) {
$characterList = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@#$%&*?"; 
$i = 0;
$salt = "";
while ($i < $max) {
    $salt .= $characterList{mt_rand(0, (strlen($characterList) - 1))};
    $i++;
}
return $salt;
}
 $user_salt = generateSalt();
$combo = $user_salt . $password; 
 //$hashed_pwd = hash('sha512',$combo);
 $hashed_pwd = md5($combo);

$sql="insert into `ice_admin_login`(ice_admin_id,username,password,salt,email,firstname,lastname,used,status,date_added) VALUES(NULL,'".$username."','".$hashed_pwd."','".$user_salt."','".$email."','".$firstname."','".$lastname."','".$used."','".$status."','".$date_added."')";

$result=mysqli_query($con,$sql);

if($result)
{
  $message="User Created Successfully";
}
else
{
  $message="Something Went Wrong";
}

}
?>

<form id="registration-form" action="" method="post">
  <div class="message"><?php if($message!="") { echo $message; } ?></div>   
   <label for="username">Username: </label><input type="text" name="username" id="username" /><br />
   <label for="password">Password: </label><input type="password" name="password" id="password" /><br />
   <label for="email">Email: </label><input type="email" name="email" id="email" /><br />
   <label for="firstname">Firstname: </label><input type="text" name="firstname" id="firstname" /><br />
   <label for="lastname">Lastname: </label><input type="text" name="lastname" id="lastname" /><br />
   <input type="submit" id="register-button" name="register" value="Register"/>
</form>