PHP Page Records display logic

On single page, 开发者_JAVA百科I am displaying 20 records at a time and with that records, I am generating PDF with their profile to export / download for admin.

Now, what i want is, for ex. on 4th Page i am passing, 4 while actual records i will have will be 61 ~ 80.. and on..

for example:

3rd page: 41 ~ 60 records
4th page: 61 ~ 80 records
5th page: 81 ~ 100 records and so on..

Does Any one have idea, How can i pass dynamic Start and End query limit in this case?

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Simple (don't consider following as valid PHP code! Because.... it's not):

$page = $_GET['page'] ? $_GET['page'] : 1;
$records_on_page = X;
$offset = $records_on_page * ($page-1);
$limit = "LIMIT $offset,$records_on_page"

page, offset, on_page
1, 00, 20
2, 20, 20
3, 40, 20
4, 60, 20
5, 80, 20
etc.

all logic.