Cannot use wp_is_mobile() in Network activated plugins

[promz]

Steps to reproduce

• Create a network site
• Create and activate a simple custom plugin, use wp_is_mobile() function anywhere in that plugin
• Login to the network dashboard and then activate the plugin

You will see an error:

Uncaught Error: Call to undefined function wp_is_mobile()

Looking at the source code of wp_is_mobile(), I can't see anything that would prevent it from loading earlier. Maybe it should be loaded earlier?

[SergeyBiryukov]

Hi there, welcome back to WordPress Trac! Thanks for the report.

wp_is_mobile() is located in wp-includes/vars.php, which is indeed loaded later than most other PHP files​ in wp-settings.php.

That said, it is considered a best practice in WordPress not to run any code directly on plugin inclusion, as the environment is not fully initialized yet, and run on the appropriate action instead, e.g. plugins_loaded or init.

Moving the wp_is_mobile() call in your plugin to a function or method that runs on a specific action should resolve the issue here.