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<?php
/**
 * Fix network admin URL to include the "/wp/" base
 * 
 * @see https://core.trac.wordpress.org/ticket/23221
 */
add_filter( 'network_site_url', function( $url, $path, $scheme ){
	$urls_to_fix = array(
		'/wp-admin/network/',
		'/wp-login.php',
		'/wp-activate.php',
		'/wp-signup.php',
		);
	foreach( $urls_to_fix as $maybe_fix_url ) {
		$fixed_wp_url = '/wp' . $maybe_fix_url;
		if ( false !== stripos( $url, $maybe_fix_url )
			&& false === stripos( $url, $fixed_wp_url ) ) {
			$url = str_replace( $maybe_fix_url, $fixed_wp_url, $url );
		}
	}
	return $url;
}, 10, 3 );