Description:
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While running the mongodb extension's test suite with PHP 7.2.0RC1, I noticed several new failures that appear to be related to the BC incompatible changes in https://github.com/php/php-src/blob/php-7.2.0RC1/UPGRADING#L28. Specifically:

> Casting arrays to objects (with (object) or settype()) will now convert integer keys to string property names.

One of the more concise tests that now fail is: https://github.com/mongodb/mongo-php-driver/blob/598e8e2845c5de09a3591276bcb6d328be48a697/tests/bson/bson-utcdatetime-001.phpt#L36

In this test, we create a numerically-indexed array that contains a single zval (a BSON class in this particular case, but that's not relevant) and convert it to BSON and back to PHP. The interim BSON representation is a document/object with a "0" string key. When converting back to PHP, we use add_assoc_zval() with "0" to assign the converted zval back to the output.

add_assoc_zval() ultimately calls zend_symtable_str_update() <https://github.com/php/php-src/blob/php-7.2.0RC1/Zend/zend_hash.h#L456>, which then converts our "0" string key to integer 0 via _zend_handle_numeric_str_ex(). Effectively, we end up creating an inaccessible numeric property on the object, which is exactly what the change in 7.2.0's array->object casting intended to address.

My question is whether add_assoc_zval_ex() should be changed to no longer convert numeric string keys to integer keys. I'll note that this decision would likely have to be made in add_assoc_zval_ex() because zend_symtable_str_update() is only provided the HashTable* and by that point we have no way of determining if the corresponding zval for that HashTable is an array or object type.

Alternatively, is it the mongodb extension's responsibility to detect this case an manually call zend_hash_str_update() instead of relying on add_assoc_zval()?