Throwing exceptions in a PHP Try Catch block

I have a PHP function in a Drupal 6 .module file. I am attempting to run initial variable validations prior to executing more intensive tasks (such as database queries). In C#, I used to implement IF statements at the beginning of my Try block that threw new exceptions if a validation failed. The thrown exception would be caught in the Catch block. The following is my PHP code:

function _modulename_getData($field, $table) {
  try {
    if (empty($field)) {
      throw new Exception("The field is undefined."); 
    }
    // rest of code here...
  }
  catch (Exception $e) {
    throw $e->getMessage();
  }
}

However, when I try to run the code, it's telling me that objects can only be thrown within the Catch block.

Please note that this particular code example makes no sense, as you can simply remove the try-catch and have the same result:

function _modulename_getData($field, $table) {
    if (empty($field)) {
      throw new Exception("The field is undefined."); 
    }
    // rest of code here...
}

so Exception will be thrown and can be caught or handled elsewhere.

In case you are going to write some handling code, that may result in re-throwing the exception, you have to throw the Exception object, not a string returned by the getMessage() method:

function _modulename_getData($field, $table) {
  try {
    if (empty($field)) {
      throw new Exception("The field is undefined."); 
    }
    // rest of code here...
  }
  catch (Exception $e) {
    if (some condition) {
        // do some handling
    } else {
        // the error is irrecoverable
        throw $e;
    }
  }
}

or if you have to do some recovery, as it is often used with database transactions: do the recovery and then re-throw:

  catch (Exception $e) {
      $db->rollback(); // rollback the transaction
      throw $e; // let the error to be handled the usual way
    }
  }