<?php
$string = "list page.php?cpage=1, list page.php?cpage=2, list page.php?cpage=3 thats all";
//$string = str_replace("?cpage=", "/", $string);
preg_match_all("/cpage=/", $string, $matches);
foreach($matches[0] as $value) {
echo $value . '<br />';
}
echo $string;
//hasil list page.php1, list page.php2, list page.php3 thats all
//yg diinginkan list page.php/1/, list page.php/2/, list page.php/3/ thats all