Uncaught Error: [] operator not supported for strings

[tmo26]

I'm writing this down just in case someone is still using the pagequery plugin in combination with php 7.1+ and stumbles over the same issue.

The following issue is found in in the nginx error log, after accessing a page that has a pagequery in it with sorting by date (other pagequeries are not affected):

2021/10/27 14:42:12 [error] 806#806: *1158977 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught Error: [] operator not supported for strings in /var/www/dokuwiki/lib/plugins/pagequery/inc/pagequery.php:787
Stack trace:
#0 /var/www/dokuwiki/lib/plugins/pagequery/inc/pagequery.php(609): PageQuery->_date_format()
#1 /var/www/dokuwiki/lib/plugins/pagequery/syntax.php(288): PageQuery->build_sorting_array()
#2 /var/www/dokuwiki/inc/parser/renderer.php(117): syntax_plugin_pagequery->render()
#3 /var/www/dokuwiki/inc/parserutils.php(673): Doku_Renderer->plugin()
#4 /var/www/dokuwiki/inc/parserutils.php(142): p_render()
#5 /var/www/dokuwiki/inc/parserutils.php(89): p_cached_output()
#6 /var/www/dokuwiki/inc/html.php(305): p_wiki_xhtml()
#7 /var/www/dokuwiki/inc/Action/Show.php(33): html_show()
#8 /var/www/dokuwiki/inc/template.php(99): dokuwiki\Action\Show->tplContent()
#9 [internal function]: tpl_content_core()
#10 /var/www/dokuwiki/inc/Extension/Event.php(129): call_user_func_array()
#11 /var/www/dokuwiki/inc/Extension/Event.php(195): dokuwiki\Extension\Event" while reading upstream, client: 66.249.76.229, server: openwrt.org, request: "GET /wiki/stats/created_pages HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "openwrt.org"

Root cause: PHP syntax has changed from 7.1+ onwards and now complains about declarations like $dkey = '';

Solution: change $dkey = ''; to $dkey = array(); -> error is gone

For more information on the background, just search the net for Uncaught Error: [] operator not supported for strings, and youo will get plenty results.