two functions : strlen & substr.
strlen will give you the text length, and substr can be used to extract some of it.
Quick & untested hack :
$limit = 1000; #your design limit
$tot_len = strlen($big_text)
$iter = float($tot_len/$limit)
$c = 1;
while ($c > $iter) {
$start = ($c-1) * $limit;
if ($iter == $c-1) {
$length = $tot_len % $limit;
} else {
$length = $limit;
}
$page[$c] = substr($big_text,$start,$length);
$c++;
}
