String ('A=0) is appended to the HTTP GET URL

Some of our users had some intermittent error on our website (PDOexception). Upon checking the log, I found that the string String ('A=0) are appended to the HTTP get. This is weird, because we definitely did not code this and it only happens to certain users in different pages and also not consistently.

Error log below: [error] 28960#0: *500749 FastCGI sent in stderr: "PHP message: An Error occurred while handling another error: exception 'PDOException' with message 'SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'A=0' LIMIT 1' at line 4'

while reading response header from upstream, client: 10.202.1.135, server: xxxx.com, request: "GET /http://ift.tt/2gG50r3'A=0 HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "xxxx.com"

Note: /http://ift.tt/2gG50r3 is correct URL without the 'A=0

Thanks for anyone who can give some insight on how/when the 'A=0 are appended.