[ClassLoader] fixed PHP warning on PHP 5.3

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Absolutely. So, we can switch to `0` to make it clear that the value does not matter at all.