sql injection attack, protection from

I'm new to PHP and would like to make certain that I have the basic protection for the site: Use double quotes to contain variable Use mysql_escape_string so that query is considered part of the WHERE clause. $result=mysql_query('SELECT * FROM users WHERE username="'.mysql_escape_string($_GET['username']).'"'); I'm pulling prices from a database and sending the item ID which has 4 characters (1001, 1002, etc.) Is the following unnecessary with mysql_escape_string? if (preg_match("/^\w{4,4}$/", $_GET['username'], $matches)) $result = mysql_query("SELECT * FROM items WHERE itemID=$matches[0]"); else // we don't bother querying the database echo "itemID not accepted"; Thanks