In your shell, or Terminal on a Mac, type:

wc -l filename

.. replacing filename with the actual name of the file. This should print the number of lines like so:

$ wc -l test.php 
      18 test.php
John John (304)
1 minute

If you want to find out how many characters are in a file here's how! You can also get a word count in Unix which is useful if you need to keep track of more than just individual characters.