PFA chief executive Simon Barker has urged players to report any illegal approaches regarding match-fixing to the FA.
Five players, including Campbell, have been bailed following the investigation, but Barker has warned that footballers need to be aware of the laws in order to protect themselves.
"We would say to players 'if you hear of any information or you are approached then make sure you protect yourself by telling somebody at the club or the players' union or speak to the FA," Barker told BBC Five Live.
"The regulations actually state if you are approached you have to report it to the FA. This has become a major issue and its something that goes across all nations and across all sports."