问题描述: Write a function that takes a string of braces, and determines if the order of the braces is valid. It should return true if the string is valid, and false if it's invalid. This Kata is similar to the Valid Parentheses Kata, but introduces new
<?php $a = array('a'=>1, 'b'=>0, 'c'=>NULL); echo 'a test by empty: ' , empty($a['a']) ? 'not exist' : 'exist', PHP_EOL; echo 'a test by isset: ' , isset($a['a']) ? 'exist' : 'not exist', PHP_EOL; echo 'a test by array_key_exists: ' , array_