Use WP core functions to check globals (WP_ADMIN, DOING...)

[herrvigg]

Reading directly the global WP defines such as WP_ADMIN, DOING_AJAX, DOING_CRON is a bad practice. Many checks are done just by looking if the global is defined. In theory a variable such as DOING_AJAX could be defined but set to false, which would have many undesired effects. In practice, it should not be the case because it's properly defined in WP. But this might be overridden in a nasty way in the code.

In any case, we should rather use the appropriate WP functions instead.

* WP_ADMIN -> is_admin()
* DOING_AJAX -> wp_doing_ajax() # from WP 4.7
* DOING_CRON -> wp_doing_cron() # from WP 4.8
* WP_CLI -> no function but we should check the value properly

There is another very good reason, these function can be linked to hooks such as wp_doing_cron or wp_doing_ajax, which might be set by other plugins.

[herrvigg]

Released in 3.7.1.