Given that we use a lot of in-house extensions, many of which deal particularly with strings, we suspected that the problem was with how strings were used in them. We wrote a lot of tests and conducted plenty of experiments, but didn’t get the results we expected. Finally, we asked for help from the main PHP engine developer, Dmitri Stogov.
One of his first questions was “Did you clear the cache?” We explained that we had, in fact, cleared the cache every time. At that point, we realized that the problem was not on our end, but with OpCache. We quickly reproduced the case, which helped us to replay and fix the problem within a few days. Without this fix that came out in the 7.0.4 version, it wouldn’t have been possible to put PHP7 into stable production.