This allows alternate ways to enable debugging. NONE means no
alternate methods are allowed. URL means when the keyword
SMARTY_DEBUG is found in the QUERY_STRING, debugging is enabled
for that invocation of the script. If $debugging is true, this
value is ignored.
该变量作用是允许以交替的方式启动调试设置。NONE表示不允许交替方式。URL表示当关键词SMARTY_DEBUG出现在QUERY_STRING（查询字符串）中时，针对此脚本的调用，调试被启动。如果$debugging变量是真，则忽略其调试控制变量值。