PHP: remove last comma of a multipolygon - php

Question

I have following issue:
I import WKT dynamicaly from DB into WKT Wicket Javascript library. I must do some substitutions to fit the WKT correctly. Since mysql fetches WKT AsText(SHAPE) i recive several arrays e.g. POLYGON((xxxx)),POLYGON((yyyy)) and so on.
First, I had to remove all "POLYGON" doing
$str = preg_replace('/^POLYGON/', '', $WKT[1]);
and add MULTIPOLYGON before <?php
tag in the wicket. It works.
Second, I must add comma between polygons, preicisely between "))((" brackets:
$str2 = str_replace(array('((', '))'), array('((', ')),'), $str);
It works but last comma remains what "slightly" deforms my multipolygon:
MULTIPOLYGON((xxx)),((yyy)),((zzz)),
How can I remove last comma?
I would be thankful for every regex or some other solution which can solve my problem.

Answer 1

In any string, you can remove the last X if you are sure that no X follows. So, you can use a negative lookahead: (,)(?!.*,), as seen here and replace it with empty string.
$result = preg_replace('/(,)(?!.*,)/', '', $str)
This doesn't look at the context though, it will just remove the last comma of any string, no matter where it is.

Answer 2

Thank you both - your answers were right and very helpful.
The problem was not string replacement. It was more the data fetching from DB.
Mysqli_fetch_array and mysqli_fetch_assoc return stringstringsring or arrayarrayarray for 3 rows fetched. That is why all commas were replaced.
I changed to mysqli_fetch_all ,then did minor replacements for each row (as array) and implode each one as variable. After i merged them into single variable, then I could apply your solutions. It is not sofisticated solution, but if it is packed into function it'll be fine.
($WKT = mysqli_fetch_all($result)) {
$str = preg_replace('/POLYGON/', '', $WKT[0]);
$str1 = preg_replace('/POLYGON/', '', $WKT[1]);
$str2 = preg_replace('/POLYGON/', '', $WKT[2]);
$str3 = implode($str);
$str4 = implode($str1);
$str5 = implode($str2);
$str6 = $str3 . $str4 . $str5;
$str7 = preg_replace('/\)\)/', ')),', $str6);
$str8 = rtrim($str7, ",");
echo $str8;
}