php - 无法在类[duplicate]中的变量定义中调用time（）

Question

This question already has an answer here:

• Is it possible to define a class property value dynamically in PHP? 2 answers

I have a class that I need to use php's time() function in:

<?php
class SomeClass {
    public $expiration = time() + (21 * 24 * 60 * 60); // 3 wks from now  

}

However this throws:

Parse error: syntax error, unexpected '(', expecting ',' or ';' in [...][...] on line 5

I can do this and it works:

<?php
class SomeClass {
    public $expiration = 0

    function __construct() {
        $this->expiration = time() + (21 * 24 * 60 * 60); 
    }
}

Why is it that I can't do it the first way?

</div>

Answer 1

Why is it that I can't do it the first way?

Because http://php.net/manual/en/language.oop5.properties.php:

"[properties] are defined by using one of the keywords public, protected, or private, followed by a normal variable declaration. This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated."