foo.txt

1. dfkadsjfsdfdsf
2. dsflkajsdlkfjads
3. asdlkfjlaksdfj
4. adsfklajasdlkfj
5. dsafkjadlskf
6. sdflkjadsf
7. adsfkljdsfdsfas
8. sadfkljadslkfj
9. adsfkjdslkf
10. dsfjasdöf
11. sdfkjdsf
12. dslfköödsalf
13. dsflöadsfdsf

pager.php

<?php

$itemsPerPage = 2;
$page = 7;

$content = file('foo.txt');
$index = count($content)-($itemsPerPage*$page);
$itemOnPage = 0;
if ($index < 0) {
    $itemOnPage = abs($index);
    $index = 0;
}

$data = array();
for($x = $itemOnPage; $x < $itemsPerPage; $x++) {
    $data[] = trim($content[$index++]);
}

print_r($data);