Thanks IgorG.
I checked php.ini as suggested however the gettext module was in there.
I managed to get this working by changing the version of PHP it was using to this:-
"D:\Plesk\Additional\PleskPHP54\ext"
Rather than the latest version that was already in there.
I have Plesk Onyx on Microsoft Windows Server 2012 R2
and am unable to access Horde webmail on any domain. The error that appears in my logs is :-
EMERG: HORDE Call to undefined function textdomain() [pid 24724 on line 63 of...