PHP Authentication API
Ehh, let's look at the biggest issue here: the SQL-Injection vulnerability.

$object = json_decode(file_get_contents("php://input"), true);

if (isset($object['email']) &amp;&amp; isset($object['password']) &amp;&amp; isset($object['firstName']) &amp;&amp; isset($object['lastName'])) {
    $email = $object['email'];

    $validationQuery="select * from members where email='$email'";


All I have to do is provide a bad string in that JSON for email and now I can destroy your database easy.

Solution: prepared statements.