[CWB] Change different frequency lists or corpus for key words generation

[D. Yuan]

Hi Andrew,

Thanks. It works. I can see the frequency list generated from other 
corpora.  Now the problem is that it results in a blank page when 
clicking the `calculate keywords' button.   The apache2 error log reports:

"""

[Wed Jun 05 18:50:54.345750 2019] [php7:error] [pid 7793] [client 
::1:41712] PHP Fatal error:  Uncaught Error: Call to undefined function 
mb_strtolower() in /var/www/html/cqpweb/lib/keywords-ui.php:830\nStack 
trace:\n#0 /var/www/html/cqpweb/exe/keywords.php(1): require()\n#1 
{main}\n thrown in /var/www/html/cqpweb/lib/keywords-ui.php on line 830, 
referer: http://localhost/cqpweb/ciiss_en/index.php?ui=keywords

"""

It seems that the case conversion function is undefined.

Best,

D.