Merge v1.x into v2.x

[mongodb-php-bot]

Merge new changes from v1.x into v2.x.

Commits

• PHPLIB-1597 Accept a $-prefixed string anywhere an expression is accepted (PHPLIB-1597 Accept a $-prefixed string anywhere an expression is accepted #1571): e352fb3

Resolving conflicts

To resolve any conflicts, check out the temporary branch and run the following command:

git merge v2.x

Ignoring changes

To ignore from the remote branch, first reset the temporary branch to v2.x and manually merge using the `ours` merge strategy:

git reset --hard v2.x
git merge --strategy=ours v1.x

Then, push the temporary branch to upate the pull request.