I used to have one class for one file. For example car.cs has the class car. But as I program more classes, I would like to add them to the same file. For example car.cs has the class car and the door class, etc.
My question is good for Java, C#, PHP or any other programming language. Should I try not having multiple classes in the same file or is it ok?
If you are working on a team, keeping classes in separate files make it easier to control the source and reduces chances of conflicts (multiple developers changing the same file at the same time). I think it makes it easier to find the code you are looking for as well.
As a rule of thumb, one class/one file is the way to go. I often keep several interface definitions in one file, though. Several classes in one file? Only if they are very closely related somehow, and very small (< 5 methods and members)
As is true so much of the time in programming, it depends greatly on the situation.
For instance, what is the cohesiveness of the classes in question? Are they tightly coupled? Are they completely orthogonal? Are they related in functionality?
It would not be out of line for a web framework to supply a general purpose widgets.whatever file containing BaseWidget, TextWidget, CharWidget, etc.
A user of the framework would not be out of line in defining a more_widgets file to contain the additional widgets they derive from the framework widgets for their specific domain space.
When the classes are orthogonal, and have nothing to do with each other, the grouping into a single file would indeed be artificial. Assume an application to manage a robotic factory that builds cars. A file called parts containing CarParts and RobotParts would be senseless... there is not likely to be much of a relation between the ordering of spare parts for maintenance and the parts that the factory manufactures. Such a joining would add no information or knowledge about the system you are designing.
Perhaps the best rule of thumb is don't constrain your choices by a rule of thumb. Rules of thumb are created for a first cut analysis, or to constrain the choices of those who are not capable of making good choices. I think most programmers would like to believe they are capable of making good decisions.