Have you consulted a Drupal support channel? It is possible that
Drupal 7 is not supposed to be compatible with PHP 7.

I did submit an issue to Views maintainers but no response yet. The strange thing is that PHP's 5.6 serialize() function manages to serialize the views object in a manner that allows successful unserialization, but the same function fails under PHP 7.

More information:

The error I get is this:

Notice: unserialize(): Error at offset 2101 of 3558 bytes in ***

Under HHVM I get:
Notice: Unable to unserialize: [{the string}]. Expected ':' but got 's'. in ***

The strange thing that there is no : in the specified position.

Allow me to attach the string to be unserialized and that causes the problem:

O:4:"view":55:{s:8:"db_table";s:10:"views_view";s:10:"base_table";s:4:"node";s:10:"base_field";s:3:"nid";s:4:"name";s:2:"fd";s:3:"vid";s:1:"1";s:11:"description";s:0:"";s:3:"tag";s:7:"default";s:10:"human_name";s:2:"fd";s:4:"core";s:1:"7";s:11:"api_version";N;s:8:"disabled";N;s:5:"built";b:0;s:8:"executed";b:0;s:7:"editing";b:1;s:4:"args";a:0:{}s:10:"build_info";a:0:{}s:8:"use_ajax";b:0;s:6:"result";a:0:{}s:12:"current_page";N;s:14:"items_per_page";N;s:6:"offset";N;s:10:"total_rows";N;s:17:"attachment_before";s:0:"";s:16:"attachment_after";s:0:"";s:17:"exposed_raw_input";a:0:{}s:8:"old_view";a:0:{}s:12:"parent_views";a:0:{}s:13:"is_attachment";N;s:5:"query";N;s:7:"display";a:2:{s:7:"default";O:13:"views_display":8:{s:15:"display_options";a:12:{s:5:"query";a:2:{s:4:"type";s:11:"views_query";s:7:"options";a:0:{}}s:6:"access";a:2:{s:4:"type";s:4:"perm";s:4:"perm";s:14:"access content";}s:5:"cache";a:1:{s:4:"type";s:4:"none";}s:12:"exposed_form";a:1:{s:4:"type";s:5:"basic";}s:5:"pager";a:2:{s:4:"type";s:4:"full";s:7:"options";a:1:{s:14:"items_per_page";s:2:"10";}}s:12:"style_plugin";s:7:"default";s:10:"row_plugin";s:4:"node";s:6:"fields";a:1:{s:5:"title";a:8:{s:2:"id";s:5:"title";s:5:"table";s:4:"node";s:5:"field";s:5:"title";s:5:"label";s:0:"";s:5:"alter";a:8:{s:10:"alter_text";i:0;s:9:"make_link";i:0;s:8:"absolute";i:0;s:4:"trim";i:0;s:13:"word_boundary";i:0;s:8:"ellipsis";i:0;s:10:"strip_tags";i:0;s:4:"html";i:0;}s:10:"hide_empty";i:0;s:10:"empty_zero";i:0;s:12:"link_to_node";i:1;}}s:7:"filters";a:1:{s:6:"status";a:6:{s:5:"value";i:1;s:5:"table";s:4:"node";s:5:"field";s:6:"status";s:2:"id";s:6:"status";s:6:"expose";a:1:{s:8:"operator";b:0;}s:5:"group";i:1;}}s:5:"sorts";a:1:{s:7:"created";a:4:{s:2:"id";s:7:"created";s:5:"table";s:4:"node";s:5:"field";s:7:"created";s:5:"order";s:4:"DESC";}}s:5:"title";s:2:"fd";s:11:"row_options";a:3:{s:10:"build_mode";s:6:"teaser";s:5:"links";b:1;s:8:"comments";b:0;}}s:8:"db_table";s:13:"views_display";s:3:"vid";s:1:"1";s:2:"id";s:7:"default";s:13:"display_title";s:6:"Master";s:14:"display_plugin";s:7:"default";s:8:"position";s:1:"1";}s:4:"page";O:13:"views_display":8:{s:15:"display_options";a:2:{s:5:"query";a:2:{s:4:"type";s:11:"views_query";s:7:"options";a:0:{}}s:4:"path";s:3:"ooo";}s:8:"db_table";s:13:"views_display";s:3:"vid";s:1:"1";s:2:"id";s:4:"page";s:13:"display_title";s:4:"Page";s:14:"display_plugin";s:4:"page";s:8:"position";s:1:"2";}}s:12:"style_plugin";N;s:13:"style_options";N;s:9:"row_index";N;s:12:"override_url";N;s:13:"override_path";N;s:13:"base_database";N;s:5:"field";N;s:8:"argument";N;s:4:"sort";N;s:6:"filter";N;s:12:"relationship";N;s:6:"header";N;s:6:"footer";N;s:5:"empty";N;s:5:"table";s:10:"views_view";s:4:"type";s:6:"Normal";s:11:"export_type";i:1;s:6:"locked";b:0;s:19:"localization_plugin";O:30:"views_plugin_localization_none":9:{s:9:"translate";b:0;s:14:"export_strings";a:0:{}s:4:"view";R:1;s:7:"display";N;s:11:"plugin_type";s:12:"localization";s:11:"plugin_name";s:4:"none";s:7:"options";a:0:{}s:10:"definition";a:11:{s:5:"title";s:4:"None";s:4:"help";s:42:"Do not pass admin strings for translation.";s:7:"handler";s:30:"views_plugin_localization_none";s:10:"help topic";s:17:"localization-none";s:6:"module";s:5:"views";s:10:"theme path";s:29:"sites/all/modules/views/theme";s:10:"theme file";s:9:"theme.inc";s:4:"path";s:31:"sites/all/modules/views/plugins";s:4:"file";s:34:"views_plugin_localization_none.inc";s:6:"parent";s:6:"parent";s:4:"name";s:4:"none";}s:9:"is_plugin";b:1;}s:7:"changed";b:1;s:15:"changed_display";a:1:{s:4:"page";b:1;}}

I tried to unserialize your string <https://3v4l.org/EYtah>, but failed in all php version, including php5.6. Is there any invisible characters in your original string?

For some reason the serialized string is corrupt. Both
view_display objects declare to have 8 properties, but they only
have 7. The views_display object declares to have 55 properties,
but it has only 51. Fixing these issues manually lets
unserialize() succeed, see <https://3v4l.org/Pvr91>.

So obviously, this is not an issue with unserialize(), but rather
with serialize(). It would be helpful to have a minimal reproduce
script.

If I were to venture a guess, it's because there is an object with a dynamic properties HT, which contains and INDIRECT to and UNDEF. The hashtable count will then include the UNDEF, but it will not be actually present in the properties list.

Here we go: https://3v4l.org/KiLqN

class A { public $b; }
$a = new A;
var_dump($a); // force properties HT
unset($a->b);
var_dump(serialize($a));

Results in "O:1:"A":1:{}" while it should be "O:1:"A":0:{}".

Thanks very much to you all for looking into this issue.

So do you think this is a PHP 7 bug or does the Drupal Views code need to be adjusted for PHP 7?

The error appears only under PHP 7.

@nikic@php.net, would you please explain this part from your comment:

"t's because there is an object with a dynamic properties HT, which contains and INDIRECT to and UNDEF" ..

The View's class definition is here (expand View source item in page):
https://api.drupal.org/api/views/includes%21view.inc/7

The ctools_object_cache code (where the view's object gets serialized):
http://www.drupalcontrib.org/api/drupal/contributions%21ctools%21includes%21object-cache.inc/7

Thanks.

@pagescene: This is definitely a bug in PHP 7. You shouldn't change the Drupal code for this ;)

The problem is basically that in PHP 7 count($array) is no longer precise (i.e. does not match the actual number of elements in the array) in some very limited circumstances. One such case is doing something like count($object->getAllProperties()) if there exists some property that was declared on the class and later unset(). In this case the count() will be reported to be one larger than it really is.

@dmitry: Could you take a look at this? Is there some way to fix this short of doing a full table scan beforehand (or serializing the contents into a separate buffer first and counting elements while doing so)?

Thanks very much @nikic & @dmitry. I can confirm that the latest commits to php-src fixed the issue. Drupal Views works flawlessly now. All the best :-)