Wordpress - restore_current_blog() vs switch_to_blog()

After every instance of switch_to_blog() you need to call restore_current_blog() otherwise WP will think it is in a "switched" mode and can potentially return incorrect data.

If you view the source code for both functions you will see those functions push/pop data into a global called $GLOBALS['_wp_switched_stack']. If you do not call restore_current_blog() after every switch_to_blog(), $GLOBALS['_wp_switched_stack'] will be non-empty. If $GLOBALS['_wp_switched_stack'] is non-empty WP thinks it is in a switched mode, even if you switched back to the original blog using switch_to_blog(). The switched mode function is ms_is_switched() and it affects wp_upload_dir(). If wp_upload_dir() thinks it is in a switched mode, it can return data that is incorrect. wp_upload_dir() builds URLs for the site, so it is a very critical function.

This is the correct use:

 foreach( $blog_ids as $blog_id ){
    switch_to_blog( $blog_id );
    //Do stuff
    restore_current_blog();
 }

If you want to run over multiple blogs there is no need to restore the previous blog each time. The only thing that grows is $GLOBALS['_wp_switched_stack'] – an array with blog IDs, nothing to worry about.

But keep in mind, restore_current_blog() will not work ( ! ! ! ) anymore after the second switch, because it uses the previous blog – which is not the first blog then. So store the first blog ID, and call …

switch_to_blog( $first_blog_id ); 
unset ( $GLOBALS['_wp_switched_stack'] );
$GLOBALS['switched'] = false; 

… instead of restore_current_blog() when you are done. The global variables must be reset, or you will run into the issues mentioned by @user42826.

The performance impact is huge. I’ve run some tests on a local installation with 12 sites:

$sites = wp_get_sites();

print '<pre>' . count( $sites ) . " sites\n";

timer_start();

print 'With restore_current_blog():    ';

foreach ( $sites as $site ) {
    switch_to_blog( $site[ 'blog_id' ] );
    restore_current_blog();
}

timer_stop( 1, 9 );

print "\nWithout restore_current_blog(): ";

timer_start();

$current_site = get_current_blog_id();

foreach ( $sites as $site ) {
    switch_to_blog( $site[ 'blog_id' ] );
}

switch_to_blog( $current_site );
$GLOBALS['_wp_switched_stack'] = array();
$GLOBALS['switched']           = FALSE;

timer_stop( 1, 9 );

print '</pre>';

Result:

12 sites
With restore_current_blog():    0.010648012
Without restore_current_blog(): 0.005203962

Using restore_current_blog() after each switch doubles the time that is needed just for switching.