Description:
------------
Completely standard PHP install on the latest PHP 7.4.9 version and we're experiencing issues with serialize() returning strings that can't be unserialized().

Ever occurrence we've seen have been from a single object, but we can't find anything specific about object itself.

All malformed strings returned match the following format:
r:[0-9]{3};

Examples include:
r:453;
r:501;
r:549;

When trying to unserialize, we get the following error:
unserialize(): Error at offset 6 of 6 bytes

We've just recently migrated to PHP 7.4 from 7.2 and had never observed this behaviour previously and have tested the same logic there since with no issue apparent so this appear to be a bug that's come in since PHP 7.2.27.

I'd try and show you an example of the object, but that's a little tricky when I can't serialize it - sorry!

Test script:
---------------
I don't expect this to help, but as I'm not able to serialize an example of the objects failing, here's a print_r() of it.

field_string Object ( [minlen] => 0 [maxlen] => 255 [raw] => [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => title [required] => 1 [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object ( [cms_table] => cms_table Object ( [id_name] => mid [table_mid] => 208 [table_name] => membership [soft_delete] => [module_title] => Memberships [module_name] => membership [shared_module] => 0 [shared_modules] => Array ( ) [database] => [has_cms_fields] => 1 [module_title_singular] => Membership [title_field] => title [publish_dates] => 0 [linked_mid] => 0 [shadow_mid] => 0 [live_field:protected] => live [deleted_field:protected] => deleted [show_validation_error_messages_above] => [show_legends] => [id] => [attributes] => Array ( ) [bootstrapped] => [legend_tag] => [enc_type] => [has_submit] => [submit_label] => [submit_class] => [submit_in_fieldset] => [pre_fields_text] => [post_fields_text] => [pre_submit_text] => [post_submit_text] => [validation_errors] => [validation_err_msg] => [show_labels_as_placeholder] => [remove_unset_fields_from_form] => [legend_attributes] => [maximum_errors_to_show] => [fields:protected] => Array ( [title] => field_string Object *RECURSION* [fn] => field_filename Object ( [minlen] => 0 [maxlen] => 255 [raw] => [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => fn [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Filename [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 2 [ftid] => filename [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => fn [title] => Filename [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => ) ) [points] => field_int Object ( [min] => 0 [max] => 9223372036854775807 [show_zero] => [allow_null] => 1 [add_class_core_field_int] => 1 [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => points [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Points [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 3 [ftid] => integer [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => points [title] => Points [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => ) ) [perks] => field_json Object ( [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => perks [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Perks [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 4 [ftid] => json [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => perks [title] => Perks [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => { "fields": [ { "title": "Perk", "type": "string" } ] } ) ) [image_5] => field_image Object ( [fid] => 5 [types] => Array ( [cms] => jpg [t] => png [s] => png ) [image_module] => membership [uploadify] => [uploadify_options] => Array ( ) [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => image_5 [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Image [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 5 [ftid] => image [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => [title] => Image [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => [image_types] => Array ( [cms] => jpg [t] => png [s] => png ) ) ) [monthly_cashback_percentage] => field_float Object ( [min] => 0 [max] => -1 [decimal_places] => 2 [allow_null] => 1 [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => monthly_cashback_percentage [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Monthly cashback percentage [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 6 [ftid] => dec [ulid] => 2 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => monthly_cashback_percentage [title] => Monthly cashback percentage [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => ) ) ) [select_fields:protected] => Array ( ) [multi_select_fields:protected] => Array ( ) [image_fields:protected] => Array ( [image_5] => field_image Object ( [fid] => 5 [types] => Array ( [cms] => jpg [t] => png [s] => png ) [image_module] => membership [uploadify] => [uploadify_options] => Array ( ) [value] => [values] => [hidden] => [in_form] => [disabled] => [readonly] => [pre_label_text] => [pre_text] => [post_text] => [tooltip_message] => [default] => [class] => [id] => [attributes] => Array ( ) [wrapper_tag] => [li_attributes] => [name] => image_5 [required] => [in_database] => 1 [fieldset] => [is_form_field] => 1 [parent_form] => membership Object *RECURSION* [label_title] => Image [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 5 [ftid] => image [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => [title] => Image [required] => [editable] => 1 [searchable] => 0 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => [image_types] => Array ( [cms] => jpg [t] => png [s] => png ) ) ) ) [fast_form_submission_spam_flag_time:protected] => 2000 [class] => form_core_default cms_table cf ) [get_titles] => [get_tables] => [get_linked_members] => [retrieve_deleted] => [retrieve_unlive] => [auto_typecast_field_validation_errors] => [is_deleted] => [is_live] => 1 [was_deleted] => [was_live] => 1 [was_id] => 3 [was_parent_id] => 0 [ts] => 2020-07-03 08:05:26 [cms_time_from] => [cms_time_to] => [table_id_set] => [has_cms_fields:protected] => [local_field_copy:protected] => [original_values:protected] => Array ( [title] => Silver [fn] => silver [points] => 50000 [perks] => {"0":{"cms_field_1":"2% Monthly cashback"},"1":{"cms_field_1":"Exclusive Discount code every month"}} [image_5] => stdClass Object ( [_] => stdClass Object ( [cdn] => 1 [w] => 190 [h] => 190 [t] => png [ts] => 1593759926 ) [cms] => stdClass Object ( [cdn] => 1 [w] => 130 [h] => 130 [t] => jpg [ts] => 1593759926 [tr] => ) [t] => stdClass Object ( [cdn] => 1 [w] => 100 [h] => 100 [t] => png [ts] => 1593759926 [tr] => 1 ) [s] => stdClass Object ( [cdn] => 1 [w] => 190 [h] => 190 [t] => png [ts] => 1593759926 [tr] => 1 ) ) [monthly_cashback_percentage] => 2 [ts] => 2020-07-03 08:05:26 [cms_created] => 2020-04-01 08:36:11 [parent_mid] => 0 [mid] => 3 ) [show_validation_error_messages_above] => [show_legends] => [id] => [attributes] => Array ( ) [bootstrapped] => [legend_tag] => [enc_type] => [has_submit] => [submit_label] => [submit_class] => [submit_in_fieldset] => [pre_fields_text] => [post_fields_text] => [pre_submit_text] => [post_submit_text] => [validation_errors] => [validation_err_msg] => [show_labels_as_placeholder] => [remove_unset_fields_from_form] => [legend_attributes] => [maximum_errors_to_show] => [fields:protected] => [select_fields:protected] => [multi_select_fields:protected] => [image_fields:protected] => [fast_form_submission_spam_flag_time:protected] => 2000 [class] => form_core_default membership cf [module_name] => membership [mid] => 3 [title] => Silver [fn] => silver [points] => 50000 [perks] => {"0":{"cms_field_1":"2% Monthly cashback"},"1":{"cms_field_1":"Exclusive Discount code every month"}} [image_5] => stdClass Object ( [_] => stdClass Object ( [cdn] => 1 [w] => 190 [h] => 190 [t] => png [ts] => 1593759926 ) [cms] => stdClass Object ( [cdn] => 1 [w] => 130 [h] => 130 [t] => jpg [ts] => 1593759926 [tr] => ) [t] => stdClass Object ( [cdn] => 1 [w] => 100 [h] => 100 [t] => png [ts] => 1593759926 [tr] => 1 ) [s] => stdClass Object ( [cdn] => 1 [w] => 190 [h] => 190 [t] => png [ts] => 1593759926 [tr] => 1 ) ) [monthly_cashback_percentage] => 2 [cms_created] => 2020-04-01 08:36:11 [parent_mid] => 0 [do_not_serialise] => Array ( [0] => cms_table [1] => select_fields [2] => multi_select_fields [3] => image_fields ) ) [label_title] => Title [show_label] => 1 [show_required_asterix] => [error_label] => [validation_error_prefix] => [source_data] => table_field Object ( [mid] => 208 [fid] => 1 [ftid] => str [ulid] => 3 [linkmid] => 0 [linkfid] => 0 [fieldset] => [name] => title [title] => Title [required] => 1 [editable] => 1 [searchable] => 1 [tablename] => membership [default] => [hint] => [source_table] => [shadow_data] => 0 [options] => ) )

Expected result:
----------------
The object serializes to a string and unserializes again to return it back to its original state. 

Actual result:
--------------
We get a malformed string that can't be unserializsed.