错误与异常2-16编程练习(未完成)

<?php
header("Content-Type:text/html;charset=utf-8");
 class MyException extends Exception {  
    public function __toString() {  
        return __CLASS__.‘[‘.$this->code.‘]:<br>‘.$this->message.‘in file‘.$this->getFile() .‘ on line ‘.$this->getLine().‘<br>‘;  
    }  
}  
//自定义函数，在何时抛出异常需自己声明  
class TestException {   
    const THROW_NONE = 0;  
    const THROW_CUSTOM = 1;  
    const THROW_DEFAULT = 2;  
 function __construct($avalue = TestException::THROW_NONE) {  
        //这里怎么写逻辑抛出异常try catch会捕获  
 }  
} 
try {  
    $o = new TestException(TestException::THROW_CUSTOM);  
} catch (MyException $e) {      // 捕获异常  
    echo "Caught my exception: <br>", $e;  
} catch (Exception $e) {        // 被忽略  
    echo "Caught Default Exception: <br>", $e;  
}  
?>