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    copied!<p>When declaring a class constant or property in PHP you can only specify a primitive values for default values. So for instance, this class declaration won't work:</p> <pre><code>class TEST { const ABC = 2 * 4; const DEF = some_function(); static $GHI = array( 'key'=&gt; 5 * 3, ); } </code></pre> <p>But this class declaration will:</p> <pre><code>class TEST { const ABC = 8; static $GHI = 15; } </code></pre> <p>These rules apply to <em>default values for class constants/properties</em> - you can always initialize other variables with the results of an expression:</p> <pre><code>$a= array( 'a'=&gt; 1 * 2, 'b'=&gt; 2 * 2, 'c'=&gt; 3 * 2, ); </code></pre> <p>The reason for this class declaration behavior is as follows: expressions are like verbs. They <em>do something</em>. Classes are like nouns: they <em>declare something</em>. A declarative statement should never produce the side-effects of an action statement. Requiring primitive default values enforces this rule. </p> <p>With this in mind we can refactor the original class as follows:</p> <pre><code>class SDK { static protected $_types= null; static public function getType($type_name) { self::_init_types(); if (array_key_exists($type_name, self::$_types)) { return self::$_types[$type_name]; } else { throw new Exception("unknown type $type_name"); } } static protected function _init_types() { if (!is_array(self::$_types)) { self::$_types= array( 'STRING_NONE'=&gt; 1 &lt;&lt; 0, // ... rest of the "constants" here 'STRING_HOSTS'=&gt; 1 &lt;&lt; 6 ); } } function __construct($fString = null) { if (is_null($fString)) { $fString= self::getType('STRING_NONE') &amp; self::getType('STRING_HOSTS'); } var_dump($fString); } } $SDK &amp;= new SDK(SDK::getType('STRING_HOSTS')); </code></pre>
 

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