＃符号用于变量和常量

Question

I have seen in many places in php that # sysmbol is used. whats the use of this and is it good to use # in constants and sql query or echo statements. I am writing some examples below:

define("LOG_TABLE", '#logger_table');

SELECT
                l.id
            FROM
                #listings l,
                #users u
            WHERE
                u.gid = '$id'

Can you please guide me the use of # symbols here.