Sending data to the database using PHP question

<?php  
$pageTitle = "Sign Up";

$pageCategory = "Sign Up";
$pageCategoryurl = "/signup.php";


//signup.php   
include($_SERVER["DOCUMENT_ROOT"] . "/inc/database3.php");
include($_SERVER["DOCUMENT_ROOT"] . "/inc/header.php"); 
include($_SERVER["DOCUMENT_ROOT"] . "/inc/search.php");

echo '<div class="ccontent">';

echo '<h3>Sign up</h3>'; 

if($_SERVER['REQUEST_METHOD'] != 'POST')  
{  
    /*the form hasn't been posted yet, display it 
      note that the action="" will cause the form to post to the same page it is on */  
    echo '<form method="post" action="">  
        Username: <input type="text" name="username" /><br>  
        Password: <input type="password" name="userpass"><br>  
        Password again: <input type="password" name="user_pass_check"><br>  
        E-mail: <input type="email" name="email"><br>  
        <input type="submit" value="Add category" />  
     </form>'; 
} 
else 
{ 
    /* so, the form has been posted, we'll process the data in three steps:  
        1.  Check the data  
        2.  Let the user refill the wrong fields (if necessary)  
        3.  Save the data   
    */  
    $errors = array(); /* declare the array for later use */  


    if(isset($_POST['username']) && !empty($_POST['username']))
    {  
        //the user name exists  
        if(!ctype_alnum($_POST['username']))  
        {  
            $errors[] = 'The username can only contain letters and digits.';  
        }  
        if(strlen($_POST['username']) > 70)  
        {  
            $errors[] = 'The username cannot be longer than 70 characters.';  
        }  
        if(strlen($_POST['username']) < 5)  
        {  
            $errors[] = 'The username cannot be shorter than 5 characters.';  
        }  

    }  
    else  
    {  
        $errors[] = 'The username field must not be empty.';  
    }  


    if(isset($_POST['userpass']) && !empty($_POST['userpass']))  
    {  
        if(!ctype_alnum($_POST['userpass']))  
        {  
            $errors[] = 'The password can only contain letters and digits.';  
        }  
        if(strlen($_POST['userpass']) > 20)  
        {  
            $errors[] = 'The password cannot be longer than 20 characters.';  
        }  
        if(strlen($_POST['userpass']) < 6)  
        {  
            $errors[] = 'The password cannot be shorter than 6 characters.';  
        }
        if($_POST['userpass'] != $_POST['user_pass_check'])  
        {  
            $errors[] = 'The two passwords did not match.';  
        }  
    }  
    else  
    {  
        $errors[] = 'The password field cannot be empty.';  
    }  

    if(!empty($errors)) /*check for an empty array, if there are errors, they're in this array (note the ! operator)*/  
    {  
        echo 'Uh-oh.. a couple of fields are not filled in correctly..'; 
        echo '<ul>'; 
        foreach($errors as $key => $value) /* walk through the array so all the errors get displayed */ 
        { 
            echo '<li>' . $value . '</li>'; /* this generates a nice error list */ 
        } 
        echo '</ul>'; 
    } 
    else 
    { 
        //the form has been posted without, so save it 
        //notice the use of mysql_real_escape_string, keep everything safe! 
        //also notice the sha1 function which hashes the password 

        $sql = mysqli_query($con,"INSERT INTO Persons (username, userpass, email)
        VALUES(     '" . mysqli_real_escape_string($_POST['username']) . "', 
                    '" . sha1($_POST['userpass']) . "', 
                    '" . mysqli_real_escape_string($_POST['useremail']) . "'");

        $result = mysql_query($sql);  
        if(!$result)  
        {  
            //something went wrong, display the error  
            echo 'Something went wrong while registering. Please try again later.'; 
            //echo mysql_error(); //debugging purposes, uncomment when needed 
        } 
        else 
        { 
            echo 'Successfully registered. You can now <a href="/signin.php">sign in</a> and start posting! :-)'; 
        } 
    } 
} 




echo '</div>';  
include($_SERVER["DOCUMENT_ROOT"] . "/inc/footer.php");
?>