Fix#1310, which was closed as stale in 2020, but which I will now reopen. Supersedes PR #1311 (@jaiminmoslake7020), from which I will remove the stale label but leave closed. The issue and the PR were too limited - they detected that the use of two equal signs at the start of a string made for an invalid formula, but there are variations, trivial and otherwise, which might also be detected. Using `setValue` with a string which starts with an equal sign will now attempt to parse (not evaluate) the formula; for certain situations in which the parser throws an exception, the string will be treated as a string rather than a formula. An example where it will still be treated as a formula is a 3D range reference, where the problem is not that it can't be parsed, but rather that the formula isn't supported (see unit test Calculation/Engine/RangeTest::test3dRangeEvaluation). Allowing such a formula might cause problems later on, but that is already what happens.
A string beginning with an equal sign but which isn't treated as a formula will automatically set the `quotePrefix` attribute to `true`; all other `setValue` attempts will set it to `false`. This avoids the problem of a lingering value causing problems later on.
It has long been a matter of discontent that setting a style can change the selected cells. A new method is added to `Worksheet`:
```php
applyStylesFromArray(string $coordinate, array $styleArray)
```
This will attempt to guarantee that the active sheet in the current spreadsheet, and the selected cells in the current worksheet, remain undisturbed after the call. The setting of `quotePrefix` above is the first use of the new method.
While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
oleibman
Adrien Crivelli
MarkBaker
Gianni Genovesi