Libs(PHP): check timestamp is not float in sign()

We've got a unit test that checks to see if an exception is thrown when
the given timestamp is a float.

The test case (which is totally fair) uses an input ending in `.0` which
casts cleanly enough to an int with the existing "is_positive_integer"
check such that the exception isn't thrown.

Adding a call to `is_float()` to this check gets us back to the test
passing.

N.b. the `+ 0` there is to cast the `$timestamp` variable to an actual
numeric type instead of a string for the purpose of the call.

Author: svix-onelson

php/src/Webhook.php

@@ -70,7 +70,7 @@ public function verify($payload, $headers)
 
     public function sign($msgId, $timestamp, $payload)
     {
-        $is_positive_integer = is_numeric($timestamp) && (int) $timestamp == $timestamp && (int) $timestamp > 0;
+        $is_positive_integer = is_numeric($timestamp) && !is_float($timestamp + 0) && (int) $timestamp == $timestamp && (int) $timestamp > 0;
         if (!$is_positive_integer) {
             throw new Exception\WebhookSigningException("Invalid timestamp");
         }