Question

I am defining a class which has a DateTime object as one of the properties. I want to set its default value to unix timestamp of ‘-1’. I also want this default value to be constant so that all objects know its value.

But I do not want to declare something like const PROPERTY_DEFAULT_DATE = '-1'; as the property will be a DateTime object and operations/functions using this default value and the property will be difficult to handle if PROPERTY_DEFAULT_DATE is not a proper DateTime object

So, can I have particular object instance of a class as constant inside another class?

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The PHP manual says

The value must be a constant expression, not (for example) a variable,
a property, a result of a mathematical operation, or a function call.

Doesn’t mention about this, but I think it can’t be done (tried lot of variations, always got syntax errors)
If it’s not possible, what alternatives do I have?

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Edit : I think I need to find the solution to the problem of defining a “Default Value” to my property, which in this case happens to be a DateTime object.
What default value will you set in the __construct()? (no, not NULL please, I expect something more elegant solution must exist) Also keep in mind that it may be used in operations/functions inside the class/subclass

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Update : I followed the advice here and created a private $_DEFAULT_DATE property and a getter for it. There is no setter for this property, so I can be assured that it’ll not be changed. (Of course, I take care not to change it within class implementation)

Answer 1

Well, unfortunately, the manual is right. You cannot put an object in a constant. You can make it a property, or in your case a static function might be suited;

YourClass::getDefaultDate(); // return DateTime('-1');