Check for duplicate user account and email address - php

Question

I am trying to check to see if a username or email address already existed in my database. I can currently connect to my database and retrieve data from my registering form. However, my SQL statements are not able to check my database to see if the user exists or not. Any help is appreciated, thanks.
<?php
require 'database.php';
$conn = Connect();
if ($conn == true){
echo "Successfully connected to database. <br><br>";}
$username = $_POST['screenname'];
$password = $_POST['password'];
$email = $_POST['email'];
echo "$username<br>"; //just checkin if it can grab data
echo "$password<br>";
echo "$email<br>";
$sql=mysql_query("SELECT * FROM users WHERE screenname = '$username'");
if(mysql_num_rows($sql)>=1)
{
echo "Username already exists";
}
else
{
//insert query goes here
}
$sql2=mysql_query("SELECT * FROM users WHERE email = '$email'");
if(mysql_num_rows($sql2)>=1)
{
echo "Email already exists";
}
else
{
//insert query
}
?>

Answer 1

Logic, programming and security (SQL injection attack) bugs fixed:
<?php
require 'database.php';
$conn = Connect();
if ($conn == true){
echo "Successfully connected to database. <br><br>";
}
$username = $_POST['screenname'];
$password = $_POST['password'];
$email = $_POST['email'];
echo "$username<br>"; //just checkin if it can grab data
echo "$password<br>";
echo "$email<br>";
$sql= sprintf("SELECT * FROM users WHERE screenname='%s' OR email='%s'",
mysql_real_escape_string($username),
mysql_real_escape_string($email)
);
if(mysql_num_rows($sql)>=1)
{
echo "Username or e-mail already exists";
}
else
{
//insert query goes here
}