Undefined function wp_set_password
When your plugin loads, pluggable functions aren’t loaded yet, in fact a lot of stuff is not loaded yet, this what actions are for. Hook your function to an action, like plugins_loaded or init, when the WP environment is loaded and initialized: add_action( ‘init’, ‘wpa80246_init’ ); function wpa80246_init(){ wp_set_password( ‘newpass’, $user_id ); }