What to do if php7 static variables cannot be assigned to arrays

With the release of PHP 7, many new features and improvements bring better performance and readability. However, when using static variables, we may encounter a problem that cannot be assigned to the array. In this article, we’ll take a closer look at this problem and how to fix it.

Before PHP 7, we could store any type of value in static variables, including arrays. For example:

function test() {
    static $arr = array('a', 'b', 'c');
    print_r($arr);
}

test(); // 输出 ['a', 'b', 'c']

However, in PHP 7, this code will throw the following error:

Fatal error: Constant expression contains invalid operations in /path/to/file.php on line 2

This is because PHP 7 has stricter restrictions on how static variables can be initialized. Now, we can only use constants to initialize static variables.

But why can’t we use arrays to initialize static variables in PHP 7? This is because in PHP 7, initialization of static variables is done at compile time, not at runtime. This means that, at compile time, PHP does not know the length of the array and the values ​​of the elements and therefore cannot determine the initial value of the static variable. In other words, PHP requires a definite constant to initialize static variables, while arrays are not a constant.

To solve this problem, we can use a class constant instead of an array. Since class constants are parsed and evaluated at compile time, they can be used to initialize static variables. For example:

class Test {
    const ARRAY = array('a', 'b', 'c');

    public function test() {
        static $arr = self::ARRAY;
        print_r($arr);
    }
}

$test = new Test();
$test->test(); // 输出 ['a', 'b', 'c']

In this example, we define a class constant named ARRAY and use it as the value to initialize the static variable $arr. We use self:: to refer to class constants because they belong to classes, not objects.

In addition to using class constants, we can also use anonymous classes introduced in PHP 7 to initialize static variables. An anonymous class is a class without a name that can be created and instantiated at runtime. They are typically used to create simple objects and closures.

The following is an example of using an anonymous class to initialize a static variable:

function test() {
    static $arr;
    if (!isset($arr)) {
        $arr = (new class {
            public $items = array('a', 'b', 'c');
        })->items;
    }
    print_r($arr);
}

test(); // 输出 ['a', 'b', 'c']

In this example, we use an anonymous class to create an object with only one attribute items . We use $arr to store the items property of this object and use it as the initial value of the static variable. Since anonymous classes are created at runtime, we can use arrays to initialize static variables.

In summary, although PHP 7 has stricter restrictions on the initialization method of static variables, we can use class constants and anonymous classes to solve the problem of being unable to assign arrays. These techniques apply not only to static variables, but also to other scenarios that require constant initial values.

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