PHP - Populate Div On Username Password Error

/* validate the username and the password */
if((!isset($_POST['username'])) || (strlen(trim($_POST['username'])) <5) || (trim($_POST['username']) != preg_replace("/[^a-zA-Z0-9\_]/", "", trim($_POST['username'])))) {	
	/* if  is bad */
	$my_error = 1;
}else{
	$username = mysql_real_escape_string(trim($_POST['username']));
}
/* END validating username */

/* validate the password */
if((!isset($_POST['password'])) || (strlen(trim($_POST['password'])) <5) || (trim($_POST['password']) != preg_replace("/[^a-zA-Z0-9\_]/", "", trim($_POST['password'])))){
	/* if  is bad */
	$my_error = 1;
}else{
	$password = trim($_POST['password']);
}
/* END validating password */

/* if any of the post variables are invalid send back to the form page */
if($my_error != 0) {	
	$_SESSION['error_message'] =$error_message;	
	header("Location: index.php");
	exit();
}


/* FUNCTION TO CREATE SALT */
function createSalt(){
	$string = md5(uniqid(rand(), true));
	return substr($string, 0, 3);
}

/* check to see if username is in the table if not send back to login */
$query01 = "SELECT * FROM registered_users WHERE username = '$username'";
$result01 = mysql_query($query01)  or die(mysql_error());

if(mysql_num_rows($result01) != 1) {	
	header("Location: index.php");	
	exit();
 }


$row = mysql_fetch_array($result01);
$salt = $row['salt'];
$hash = hash('sha256', $salt, $password);
$query02 = "SELECT id FROM registered_users WHERE username = '$username' AND password = '$hash'";
$result02 = mysql_query($query02)  or die(mysql_error());
if(mysql_num_rows($result02) !=1){ 
	header("Location: index.php");	
	exit();
}
$_SESSION['id'] = $row['id'];
$_SESSION['valid_user'] = "yes";
header("Location: admin02.php");	
exit();
?>

    /**
     * Select the database
     *
     * @access    private called by the base class
     * @return    resource
     */
    function db_select()
    {
        return @mysql_select_db($this->database, $this->conn_id);
    }

Can someone tell me why my php code isn't inserting users into the database. My database is made and table is made. I created a register form so my visitors can register but im getting a blank page.

I has something to do with the following code

$statement = $connection->prepare('INSERT INTO users (username, email, password) VALUES (:usernamebox, :emailbox, :passwordbox)');
if($statement){
$result = $statement->execute([
':usernamebox' => $data['usernamebox'],
':emailbox' => $data['emailbox'],
':passwordbox' => $data['passwordbox'],
]);

if ($result) {
$_SESSION['messages'][] = 'Thanks for registering. Check your email to confirm your email';
header('Location: register.php');
exit;
}

Why is there the " : "  before the usernamebox

Heres how my form looks like

<form action="register-clicked.php" method="POST">
Username:
<input type="text" name="usernamebox" placeholder="Enter Username Here">
Email:
<input type="text" name="emailbox" placeholder="Enter email here">Password:
<input type="password" name="passwordbox" placeholder="Enter password here">
Confirm Password:
<input type="password" name="passwordconfirmbox" placeholder="Re-enter password here">
<input type="submit" name="submitbox" value="Press to submit">
</form>

I have developed a code for a login and seems to work well (No syntax error according to https://phpcodechecker.com/ but when I enter a username and a password in the login form, I get an error HTTP 500. I think that everything is ok in the code but obviously there is something that I am not thinking about. The code (excluding db connection):

$id="''"; $username = $_POST['username']; $password = md5($_POST['password']); $func = "SELECT contrasena FROM users WHERE username='$username'";

$realpassask = $conn->query($func); $realpassaskres = $realpassask->fetch_assoc(); $realpass= $realpassaskres[contrasena];

$func2 = "SELECT bloqueado FROM users WHERE username='$username'"; $blockedask = $conn->query($func2); $blockedres = $blockedask->fetch_assoc(); $bloqueado = $blockedres[bloqueado];

//Login if(!empty($username)) { // Check the email with database
$userexists = $pdo->prepare("SELECT COUNT(username) FROM users WHERE username= '$username' LIMIT 1"); $userexists->bindParam(':username', $username); $userexists->execute(); // Get the result $userexistsres = $userexists->fetchColumn(); // Check if result is greater than 0 - user exist if( $userexistsres == 1) { if ($bloqueado == NO) { if ($password != $realpass) { die("contrasena incorrecta"); } Else{ $_SESSION['loguin']="OK"; $_SESSION['username']="$username"; header("Location: ./index.php"); } } Else{ die("Tu usuario ha sido bloqueado o todavía no ha sido aceptado por un administrador. } Else{ die("No hay ninguna cuenta con este nombre de usuario"); } } else { echo 'El campo usuario esta vacio'; } ?>