int|null $main_site_id, WP_Network $network )
Filters the main site ID.
Returning a positive integer will effectively short-circuit the function.
If a positive integer is returned, it is interpreted as the main site ID.
The network object for which the main site was detected.
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$main_site_id = (int) apply_filters( 'pre_get_main_site_id', null, $this );