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After every instance of switch_to_blog() you should call restore_current_blog() to restore the current (actually, previous) blog.

But if you're looping through two or more blogs and calling switch_to_blog() on each, is there any reason not to use an additional switch_to_blog() at the end of the loop to switch to the original blog rather than calling restore_current_blog() at each pass.

E.g.

Why not:

 $original_blog_id = get_current_blog_id();
 foreach( $blog_ids as $blog_id ){
    switch_to_blog( $blog_id );
    //Do stuff
 }
 switch_to_blog( $original_blog_id );

instead of:

 foreach( $blog_ids as $blog_id ){
    switch_to_blog( $blog_id );
    //Do stuff
    restore_current_blog_id();
 }
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Now I understand this, thanks for correcting that answer of mine ;) Am revising everything. –  brasofilo Mar 3 '13 at 10:55
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2 Answers 2

up vote 4 down vote accepted

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.

FYI, I ran into this problem using a plugin that used switch_to_blog(). The plugin did not use restore_current_blog() to restore, instead it used switch_to_blog() to switch back to the original site. With the plugin active, all my site urls that were generated were incorrect.

This is the correct use:

 foreach( $blog_ids as $blog_id ){
    switch_to_blog( $blog_id );
    //Do stuff
    restore_current_blog();
 }
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Thanks, I've not had chance to work through the soup of constants & logic that wp_upload_dir() employs to generate urls, but I shall take your word that this does indeed result in buggy behaviour. In any case, the existence of ms_is_switched() means my alternative approach results in the function not behaving as expected and could break plug-ins as well as core. Thanks –  Stephen Harris Nov 17 '13 at 22:59
1  
If this is true, then the Codex page for restore_current_blog() needs an update, since it says that for multiple switches, one need only save the current $blog_id and then use multiple switch_to_blog() calls. –  Pat J Nov 21 '13 at 16:54
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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.

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Thought there wasn't any reason not to. Was confused why restore_current_blog() didn't just retrive the previous blog ID and call switch_to_blog() - a brief look at the code source and it seems there's a bit of code duplication... –  Stephen Harris Mar 2 '13 at 20:52
    
great info. Didn´t know that . Maybe restore_current_blog() should have switched the name to restore_previous_blog(). or maybe change the whole function´s own functionality to operate as a real restore_main_blog(). trac material. –  krembo99 Apr 11 '13 at 3:52
    
One more thing needs to be set as well: $GLOBALS['switched'] = false;. Although the function ms_is_switched() do check for $GLOBALS['_wp_switched_stack'] but this is introduced since 3.5 and some plugins are still using $GLOBALS['switched'] to see if WP is in switched mode or not. –  Parham Jan 28 at 10:14
    
@Parham Thanks! I’ve added that to my answer. –  toscho Jan 28 at 10:16
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