In my version of Emacs (info "(elisp) Font Lock Basics") says
The fifth element, SYNTAX-BEGIN, specifies the value of
`font-lock-beginning-of-syntax-function'. We recommend setting
this variable to `nil' and using `syntax-begin-function' instead.
while the doc string for `font-lock-default' says
If SYNTAX-BEGIN is non-nil, it should be a function with no args
used to move backwards outside any enclosing syntactic block, for
syntactic fontification. Typical values are `beginning-of-line'
(i.e., the start of the line is known to be outside a syntactic
block), or `beginning-of-defun' for programming modes or
`backward-paragraph' for textual modes (i.e., the mode-dependent
function is known to move outside a syntactic block). If nil, the
beginning of the buffer is used as a position outside of a syntactic
block, in the worst case.
(This maybe changed in CVS, but I am unable to check now.)