The statement below is taken from the official Government website,
http://www.parliament.uk/about/how/business/questions/"Prime Minister's Question Time
The Prime Minister answers questions from MPs in the Commons for half an hour every Wednesday from Midday.
The session normally starts with a routine question from an MP about the Prime Minister's engagements. This is known as an 'open question' and means that the MP can then ask a supplementary question on any subject.
Following the answer, the MP then raises a particular issue, often one of current political significance. The Leader of the Opposition then follows up on this or another topic, they are permitted to ask a total of six questions. The Leader of the Opposition is the only MP who is allowed to come back with further questions.
Most MPs will table the same question about engagements and if they do, only their names will appear on the question book. After the first engagements question has been asked, any other MPs who have tabled the same question are simply called to ask an untabled, supplementary question. This means, in theory, that the Prime Minister will not know what questions will be asked of him.
However, the Prime Minister will be extensively briefed by government departments in anticipation of likely subjects he could be asked about."There is a big difference in "pre-planned" and "anticipation "