You need to check whether the variable is empty. !empty checks that it is NOT empty. If you want to also check for a valid email address, you can do that at the same time. Leave out the " OR !isEmail($data['abstract'])" if you don't:
PHP Code:
if(empty($data['abstract']) OR !isEmail($data['abstract'])) {
$err[] = "ERROR - Please enter a valid email";
}
if(!empty($err)) {
echo "<div class=\"msg\">";
foreach ($err as $e) {
echo "* $e <br>";
}
echo "</div>";
}