Users table structure is this: id|username|password|email

id - auto_increment + primary_key
username - unique index
email - unique index

$post_input = $_POST['user_input'] //will be username or email

$check = $db->prepare("Select * from users where username=? or email=?")
$check->execute(array($post_input,$post_input));
if($check->rowCount() > 0){
   //Username or Email found from here do the rest of the things
}

$post_input = $_POST['user_input'] //will be username

$check = $db->prepare("Select * from users where username=?")
$check->execute(array($post_input));
if($check->rowCount() > 0){
   //Username found from here do the rest of the things
}

Select * from users where username=? limit 0,1