也发现了这个问题，追了源码：
在ErrorHandler类里，有这个变量：
/**

 * @var integer the size of the reserved memory. A portion of memory is pre-allocated so that
 * when an out-of-memory issue occurs, the error handler is able to handle the error with
 * the help of this reserved memory. If you set this value to be 0, no memory will be reserved.
 * Defaults to 256KB.

*/
public $memoryReserveSize = 262144;

/**
     * Register this error handler
     */
    public function register()
    {
        ini_set('display_errors', false);
        set_exception_handler([$this, 'handleException']);
        if (defined('HHVM_VERSION')) {
            set_error_handler([$this, 'handleHhvmError']);
        } else {
            set_error_handler([$this, 'handleError']);
        }
        if ($this->memoryReserveSize > 0) {
            $this->_memoryReserve = str_repeat('x', $this->memoryReserveSize);
        }
        register_shutdown_function([$this, 'handleFatalError']);
    }

因为php没有用户主动分配内存的机制，所以，为了内存溢出也能抛出错误，预留了26w个x预先占用了内存。。。
是不是很冷，哈哈