i have this description that holds users mention tag.
[MENTION=1]one[/MENTION][MENTION=4]two[/MENTION][MENTION=748]three[/MENTION] HI, it is me!!
i need to know who is the members that are mentioned in this description,so i expect.
preg_match_all('/[MENTION=[1-9]](.*?)[\/MENTION]/s', $html, $matches);
 => one[
 => [
 => two[
 => [
 => three[
 => H
Square brackets are reserved characters in a RegEx. You need to escape them. Something like this should work:
preg_match_all('/\[MENTION=([1-9]+)\](.*?)\[\/MENTION\]/s', $html, $matches);
I've also added a capture group around the IDs,
([1-9]+), so you get the numeric values and the text-content in your matches (
$matches will contain the numbers and
$matches will contain the text-contents).