When you register at a website, the site normally checks if teh email address that you enter is in a valid format.  This is done with a Regular Expression.  What we need to do is check if a string - the email - matches the regular expresion:

$email = $_POST['email']; ereg("^[a-zA-Z0-9_]+@[a-zA-Z0-9\-]+\.[a-zA-Z0-9\-\.]+$]", $email);

At the moment that only checks the string ($email), however, it does not output anything.  So we must write an if statement that returns a boolean value if the string does not matches the regular expression.

$email = $_POST['email']; if (eregi("^[a-zA-Z0-9_]+@[a-zA-Z0-9\-]+\.[a-zA-Z0-9\-\.]+$]", $email)) { return FALSE; }

This is just a simple check to make sure the email is a valid format - that is similar (email@example.com).  You can do a more complicated check of the email by splitting the email where '@' is and check that the domain exists and online.  An example provided below:

list($emailAddy, $domain) = split("@", $email); if(fsockopen($Domain, 25, $errno, $errstr, 30)) { return true; } else { return false; }

Put the two checks together and you have a seamless email validation script =)  Enjoy