Thanks for the reply Nigel, you put your finger on the problem.
I was trying to use a beta installation of PHP 5.6.8 on free.fr (not my personal web site) and there is a list of functions which are not supported/de-activated, which includes set_include_path(). I’ll just have to hope that this restriction will be removed in the final release. I don’t suppose there’s a workaround I could use until if and when the restriction is removed? (For example hard-coding the value of WT_ROOT)