Drupal Menu Page Callback Function went wrong with arguments
Really embarrassed to say I made one tiny small mistake in this line of code:

return t('Hello, @first_name @last_name'，
array('@first_name' =&gt; $first_name, '@last_name' =&gt; $last_name));


I was typing the first comma while switched to Chinese Simplified PinYin, correct code should be: 

return t('Hello, @first_name @last_name',
array('@first_name' =&gt; $first_name, '@last_name' =&gt; $last_name));


After I changed it, everything is okay now. Thank you all for paying attention to my silly question.