Must Use Plugins are loaded without a manual activation. They are stored in wp-content/mu-plugins and don’t need a regular plugin header.

mu stands for must use, not for . These plugins are stored in wp-content/mu-plugins by default, but one can change that in with two constants: WPMU_PLUGIN_DIR and WPMU_PLUGIN_URL.

Mu-plugins are loaded in alphabetical order for all sites in a network or a single site installation on each request before the regular plugins.

The first action after all mu-plugins are loaded is 'muplugins_loaded'.


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