If you are experiencing an issue with how $readini() works in the latest version of mIRC, please provide us with an example script that reproduces the issue. For example:
var %fn = test.ini
writeini %fn section item value
var %value = $readini(%fn,section,item)
echo value: %value
Also note that mIRC v6.16 was released in 2004, over fourteen years ago. If you must use an older version of mIRC, you should not under any circumstances use any version of mIRC older than mIRC v6.35. mIRC v6.35 includes security updates that fix remote exploits existing in older versions of mIRC. You can download mIRC v6.35 through the download page on the mIRC website.
While new versions of mIRC are updated in a way that tries to maintain backwards compatibility, sometimes that is not possible and if a script is very old, be aware that you may need to udpate it to work with newer versions of mIRC.