I have a bit of a workaround. It unfortunately is quite a bit slower, but is tested OK on an office 365 server. Retrieve a list of the mailboxes first using imap_getmailboxes, then for each of those items in the array of mailboxes, use imap_reopen to switch your imap stream to the new mailbox. imap_num_msg will return the number of messages in your imap stream, which you can save to a variable, then switch back to the original mailbox using imap_reopen again. The below function is similar to imap_status, but will return only the number of messages rather than the full mailbox information.

function my_imap_status($stream, $mailbox='', $info=SA_ALL)
    {
        // get current mailbox name (and info)
        $curr_obj=imap_check($stream);
        if(!$curr_obj) return false;
        
        // if request if for current mailbox then just return it
        if( (empty($mailbox)) || ($mailbox==$curr_obj->Mailbox) ) return $curr_obj;
        
        // get current mailbox
        $current_mailbox=$curr_obj->Mailbox;
        
        // switch to new mailbox
        if(!imap_reopen($stream, $mailbox)) return false;
        
        // get number of messages
        $numMessages = imap_num_msg($stream);

        // switch back to original mailbox
        imap_reopen($stream, $current_mailbox);
        
        // return info
        return $numMessages;
    }

        $list = imap_getmailboxes($mailbox, $serverString, "*"); 
	    if (is_array($list)) { 
	        foreach ($list as $key => $val) {
	           $mapname = str_replace($serverString, "", $val->name);
	           $sstring = rtrim($serverString, "*");
	           $foldername = str_replace($sstring, "", $val->name);
	           //$mbox_info = imap_status($mailbox, $val->name, SA_ALL);
	           //$numMessages = $mbox_info->messages;
	           $numMessages = my_imap_status($mailbox, $val->name, SA_ALL);
			   
	           if ($mapname != ".") {
		       $list_folders[$key]['serverstring'] = $sstring;
		       $list_folders[$key]['folder'] = $val->name; 
	               $list_folders[$key]['name'] = $mapname; 
	               $list_folders[$key]['delimit'] = $val->delimiter; 
	               $list_folders[$key]['attributes'] = $val->attributes; 
	               $list_folders[$key]['number'] = $numMessages; 
	           } 
	            
	        } 
	    } else { 
	        echo "imap_getmailboxes failed: " . imap_last_error() . "\n"; 
	    }

Exchange adds extra space in the "STATUS" response, which IMAP library fails to parse. It can be fixed with a small patch in the libc-client library:

--- src/c-client/imap4r1.c.ORIG 2018-12-22 13:24:15.899707119 +0100
+++ src/c-client/imap4r1.c      2018-12-22 13:24:18.551731625 +0100
@@ -4092,7 +4092,7 @@
     unsigned char *txt = reply->text;
     if ((t = imap_parse_astring (stream,&txt,reply,&j)) && txt &&
        (*txt++ == ' ') && (*txt++ == '(') && (s = strchr (txt,')')) &&
-       (s - txt) && !s[1]) {
+       (s - txt) && !s[1] || s[1] == ' ') {
       *s = '\0';               /* tie off status data */
                                /* initialize data block */
       status.flags = status.messages = status.recent = status.unseen =

That's apparently a bug in libc-client, not in PHP's IMAP
extension.