Test by appending each directory in PHP_SELF to DOCUMENT_ROOT
when the realpath of that is the same as dirname(__FILE__) then
stop processing.

For most users there is little performance hit, only a few will
have more than say two levels above DOCUMENT_ROOT

If nothing can be autoconfigured, use a default of root (ie no
path)

Bug #780237

Change-Id: I60339a180ac043ab0b89c3a49ff5f2485112f567
Signed-off-by: Hugh Davenport <hugh@catalyst.net.nz>