This is a good question that can be answered on a number of levels:
- Language characteristics
- Test coverage
- CASE tools
1. Language characteristics As you have pointed out the characteristics of the PHP language differ markedly from the more strongly-typed languages such as Java. This raises a serious issue where programmers coming from the more strongly-typed languages such as Java and C# may not be aware of the implications of PHP's behaviour (such as those you have described). This introduces the possibility of mistakes on the part of the programmer (for example, a programmer who may have been less careful using Java because they know the compiler will catch incorrect parameters may not apply the appropriate care when developing in PHP). Consequently, better programmer education/supervision is needed to address this issue (such as in-house company coding standards, pair programming, code review). It also (as you have pointed out) raises the question of whether test coverage should be increased to check for such mistakes as would have been caught by a compiler.
2. Test Coverage The argument for test coverage is very project-specific. In the real world, the level of test coverage is primarily dictated by the error tolerance of the customer (which is dictated by the consequences of an error occuring in your system). If you are developing software that is to run on a real-time control system, then obviously you will test more. In your question you identify PHP as the language of choice; this could apply equally to the ever-increasing number of web-enabled frontends for critical systems infrastructure. On the other side of the coin, if you are developing a simple website for a model railroad club and are just developing a newsletter app then your customer may not care about the possibility of a bug in the constructor.
3. CASE Tools Ultimately it would be desirable for a CASE tool to be available which can detect these errors, such as missing parameters. If there are no suitable tools out there, why not create one of your own. The creation of a CASE tool is not out of reach of most programmers, particularly if you can hook into an open-source parsing engine for your language. If you are open-source inclined this may be a good project to kick start, or perhaps your company could market such a solution.
Conclusion In your case whether or not to test the constructors basically comes down to the question: what will the consequences of a failure in my system be? If it makes financial sense to expend extra resources on testing your constructors in order to avoid such failures, then you should do so. Otherwise it may be possible to get by with lesser testing such as pair programming or code reviews.