Use PHP 8.0 union types

[marijnvanwezel]

PHP 8.0 introduces the concept of union types, which allows us to explicitly type a function with multiple different types. php-cypher-dsl currently polyfills this feature through the ErrorTrait trait. However, there are a few problems with this polyfill approach:

• It is not understood by IDE's, which makes refactoring hard;
• It requires an additional function call;
• Type-checking is only done at the deepest call;
• It does not work for interfaces or abstract methods.

Therefore, I want to propose to drop support for PHP 7.4 and start using PHP 8.0 union types. Implementing this is relatively straightforward:

1. Add union types where needed to return types;
2. Add union types where needed to argument types;
3. Add union types where needed to class parameter types;
4. Remove any references to the ErrorTrait trait.

The ErrorTrait trait also has a method for checking whether the type of each value in an array is allowed. This is currently not supported through PHP's type system. There are two options to fix this:

• Use PHPDoc to specify which types the array allows and use PHP's native array type in signatures, or
• Keep supporting the type-checking of arrays.

I think the first option is best. Type checking of arrays is only used in a few places (five times throughout the entire library), and it adds complexity and overhead. Most usages can also be replaced by typed variadic parameters.