Wann werden statische PHP-Variablen zerstört?

/**
     * 获取所有下级    
     * $data          用户数组          
     * $pid           用户user_id
     * $level         递归层次            
     * $bool_level    判断是否分级(不分级则获取所有的下级)
     * $clear         是否销毁静态变量
     */
    public function getTree($data, $pid = 0, $level = 0, $bool_level = true ,$clear=false ) {
 
        if ($pid < 1) {
            return false;
        }
        
        static $arr = [];
        
        if($clear){
            $arr = null;
            static $arr = [];
        }
        
        if ($bool_level) {
            $level = $level - 1;
        } else {
            $level = 1;
        }
 
        if ($level >= 0) {
            foreach ($data as $key => $val) {
                if ($val[&#39;pid&#39;] == $pid) {
                    $arr[$level + 1][] = $val[&#39;id&#39;];
                    $this->getTree($data, $val[&#39;id&#39;], $level, $bool_level);
                }
            }
        }
        
        return $arr;
        
    }