In PHP when you define a class level constant as in:
const MY_CONSTANT = 'hello constant';
Why can't you initialize this value with a function such as
const MY_FILEPATH = dirname(dirname(__FILE__)) . '/heres-my-file.php';
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5Welcome to SO. This has already been discussed in depth here: stackoverflow.com/questions/3960323/… the answer is long :) Anyway, it's definitely not possible, you may want to leave it at that.Pekka– Pekka2011-04-02 22:44:51 +00:00Commented Apr 2, 2011 at 22:44
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2 Answers
In short: The constants are replaced while parsing, but functions are executed while interpreting. The parser simply cannot know to what it should set the value of the content.
1 Comment
David
Hi, thanks. Ok that kind of makes sense then. Whats the best practice for doing this kind of thing as it must happen quite a lot - where a constant value is requried but it is defined from a function call or something.
Constants are immutable. Therefore, if functions could change the value of a constant it wouldn't be a constant.
5 Comments
David
Yes but i am not trying to change the value of a constant, instead i am trying to initialise it with the value of a function.
Nabeel
I understand what you mean. However, if the function could initialize a constant, theoretically it would be able to change its value. Because the constant must be initialized in the class (if it was declared in the class too).
KingCrunch
"if the function could initialize a constant, theoretically it would be able to change its value". This doesnt make much sense to me. If a constant is initialized, its a constant. If its not initialized, it does not exists. There is never a change.
Nabeel
I'm sorry, I meant "if the function could (set) a constant value ..". Because the value of a constant must be set when declared and only once.
Jason S
your answer is not helpful. This pattern is allowed in Java for
final variables that are evaluated exactly once. Evidently it is not allowed in PHP. But the reason why has nothing to do with functions changing the value of a constant; there's no inherent reason why a variable could not be assigned once from the value of a function. If the programmer uses in this instance a function that does not return identical results for function calls with the same arguments... well, that's a poor choice on the part of the programmer to use for a definition of a constant. Otherwise, why not?