func_args.php

<?php

Class Api {
    public static $log;

    public static function setLogger($func) {
        self::$log = &$func;
    }

    public static function errorLog($st) {
        $func = &self::$log;
        $func($st);
    }

    public function doSth() {
        self::errorLog('In doSth');
    }
}


Class Logger {

    public static function log($statement) {
        printf("%s \r\n", $statement);
    }

}

Api::setLogger(function($statement){
    Logger::log($statement);
});

Api::errorLog('Hello world');
Api::errorLog('This function is set later');

$api = new Api;
$api->doSth();

//Api::$log = function ($st) {
//    print $st;
//};
//Api::$log('Hello');


?>