Walker joins Peterborough

St Mirren boss Danny Lennon is relieved to be preparing for the Scottish Communities League Cup final following a prolonged build-up.
The Buddies beat Celtic 3-2 in the semi-final in January and will face Hearts on their return to Hampden on Sunday.
Lennon's side followed their success over Celtic with consecutive victories, but have only won one game in seven since then.
And, as the St Mirren squad embark on a training break in St Andrews, Lennon is happy at last to have his charges concentrate on the first domestic final of the season.
"It's been a difficult challenge for the players to focus, more so in the last couple of weeks, and I totally understand that," Lennon said.
"So, no more distractions. They can totally focus on it, we can start to talk about it, we can build the game up and enjoy the week.
"And we will make sure there is no stone unturned in our preparation to do our best to bring this piece of silverware home.
"That's the point of going away early in the week. We will do a lot of our work in the next two or three days and at the tail end of the week we will get back to normality as quickly as possible."
The 39-year-old, recently released by Walsall after spending most of his 20-year playing career with the midlands club, has been handed an initial contract by Posh until the end of the season.
Walker made over 450 appearances for Walsall and played in 15 games for West Ham during a five-year stint at Upton Park.