filter_var(), FILTER_VALIDATE_EMAIL

The filter_var() function filters a variable with the specified filter.
Returns the filtered data on success, or FALSE on failure.
The FILTER_VALIDATE_EMAIL filter validates an e-mail address.

<?php
$email = "john.doe@example.com";

if (!filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
  echo("$email is a valid email address");
} else {
  echo("$email is not a valid email address");
}
?>