DDC-3857: [GH-1485] Changed references from PHP6 to PHP7

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of hasumedic:

Url: #1485

Message:

Found a reference to PHP6 while reading the best practices docs.

While doing this tiny change, I also found a couple more references to it in the tests which I changed too.

If this is a funny reference to it, I'm happy for you to leave it as is. Otherwise, this PR replaces the existing references to PHP6 for PHP7 instead.

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-1485] was merged:
#1485

[doctrinebot]

Issue was closed with resolution "Fixed"