How to Define Class Properties with Expression Values in PHP?

Workaround for Basic Syntax Not Being Parsed

When attempting to define a class property with an expression on the right side of the equals sign, PHP raises an error. This is because PHP only allows primitive values as default values for class constants and properties.

To bypass this limitation, we can use a two-step approach:

1. Introduce a Static Array of Constants

Define a static array $_types within the class. This array will hold all possible constant values.

<code class="php">static protected $_types = null;</code>

2. Create a Method to Retrieve Constant Values

Implement a getType() method that allows you to retrieve constant values by name.

<code class="php">static public function getType($type_name) {
    self::_init_types();
    if (array_key_exists($type_name, self::$_types)) {
        return self::$_types[$type_name];
    } else {
        throw new Exception("unknown type $type_name");
    }
}

protected function _init_types() {
    if (!is_array(self::$_types)) {
        self::$_types = [
            'STRING_NONE' => 1 << 0,
            // ... include all constants
            'STRING_HOSTS' => 1 << 6
        ];
    }
}</code>

3. Initialize Class Properties Using getType()

Within the constructor, you can now initialize class properties using the getType() method.

<code class="php">function __construct($fString = null) {
    if (is_null($fString)) {
        $fString = self::getType('STRING_NONE') & self::getType('STRING_HOSTS');
    }
    var_dump($fString);
}</code>

By utilizing this workaround, you can retain readability and future expandability while adhering to the syntactic restrictions of PHP.

Example:

<code class="php">$SDK = new SDK(SDK::getType('STRING_HOSTS'));</code>

The above is the detailed content of How to Define Class Properties with Expression Values in PHP?. For more information, please follow other related articles on the PHP Chinese website!