预定义异常
Exception是所有异常的基类，类摘要如下：
<code><?php class Exception { protected string $message; // 异常消息内容 protected int $code; // 异常代码 protected string $file; // 抛出异常的文件名 protected int $line; // 抛出异常在该文件的行号 public __construct([string $message = "" [, int $code = 0 [, Exception $previous = NULL]]]) final public string getMessage(void) final public Exception getPrevious(void) final public int getCode(void) final public string getFile(void) final public int getLine(void) final public array getTrace(void) final public string getTraceAsString(void) public string __toString(void) final private void __clone(void)}</code>ErrorException是错误异常，类摘要如下：
<code><?php class ErrorException extends Exception { protected int $severity; // 异常级别 public __construct([string $message = "" [, int $code = 0 [, int $severity = 1 [, string $filename = __FILE__ [, int $lineno = __LINE__ [, Exception $previous = NULL]]]]]]) final public int getSeverity(void)}</code>（全文完）
