Issue 67910 and more: replace php5 with php7

So I've replaced all occurences of php5 with php7 with a couple of sed.

It means:

- `find . -type f -not -path './.git/*' | xargs sed -i 's/php5/php7/g'`
- `find . name '*php5*'` and manually renaming them.

It actually wasn't so much work.

I then ran `make test`, and only a couple of tests actually failed.

Here is the list of tests that failed:

```
=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
Directly modifying an unreferenced array when foreach'ing over it while using &$value syntax. [tests/lang/foreachLoop.013.phpt]
Directly modifying a REFERENCED array when foreach'ing over it. [tests/lang/foreachLoop.014.phpt]
Directly modifying a REFERENCED array when foreach'ing over it while using &$value syntax. [tests/lang/foreachLoop.015.phpt]
GC 029: GC and destructors [Zend/tests/gc_029_zts.phpt]
Bug #67118 crashes in DateTime when this used after failed __construct [ext/date/tests/bug67118.phpt]
Regression introduce in fix for Bug #67118 - Invalid code [ext/date/tests/bug67118_2.phpt]
via [ext/pdo_sqlite/tests/common.phpt]
        SQLite PDO Common: PDO::FETCH_FUNC and statement overloading [ext/pdo_sqlite/tests/pdo_011.phpt]
Bug #64146 (serialize incorrectly saving objects when they are cloned) [ext/standard/tests/serialize/bug64146.phpt]
=====================================================================
```

I think this cleanup will have to be done eventually anyway.

I'm afraid I can't fix all these tests myself. So I'm asking for your help! The bulk of work is done in this PR, the only thing left is fixing these tests.

Related issue: https://bugs.php.net/bug.php?id=67910
How should I update this comment? "only work under PHP5+"?