Set AMP_QUERY_VAR fallback when AMP is inactive

[mjangda]

If AMP is deactivated via the amp_is_enabled filter, the AMP_QUERY_VAR ends up undefined. This causes warnings when a helper function like is_amp_endpoint() is called. Setting a fallback value prevents this.

This was happening with a site running both amp-wp (but deactivated via the filter) and lazy-load (which calls is_amp_endpoint()):

Warning: Use of undefined constant AMP_QUERY_VAR - assumed 'AMP_QUERY_VAR' (this will throw an Error in a future version of PHP) in wp-content/plugins/amp/includes/amp-helper-functions.php

To reproduce:

add_filter( 'amp_is_enabled', '__return_false' );

add_action( 'wp_head', function() {
    is_amp_endpoint();
} );

[mjangda]

We may also want to consider hardening up the helper functions as well to return appropriate values when AMP is disabled via the filter.

[westonruter]

See also #945. What have been the use cases for AMP_QUERY_VAR in the wild?

[mjangda]

One example is http://indianexpress.com/article/india/breakthrough-journalist-gauri-lankesh-murder-first-arrest-5084078/lite/ (they use lite instead of amp).

Deprecation is fine too :)

[westonruter]

We may also want to consider hardening up the helper functions as well to return appropriate values when AMP is disabled via the filter.

I think this might be the better way to go instead of defining AMP_QUERY_VAR as false, as if someone does later define AMP_QUERY_VAR then they'll get a notice about it already being defined. Or rather, if changing the slug is to be deprecated, then maybe we should just stop using AMP_QUERY_VAR altogether and instead introduce an amp_get_slug() function instead.

[westonruter]

Rebased to fix merge conflict. Former head was 82dced4.

[mjangda]

we should just stop using AMP_QUERY_VAR altogether and instead introduce an amp_get_slug() function instead.

Sounds like a good idea.

[ThierryA]

@westonruter I see this PR is now approved, does it still need my review?

[westonruter]

@ThierryA I just wanted you to be aware of it.