It would be nice to have an index for the php manual.
Thank you.

Could you be more specific in what you mean by an 'index'?

http://www.php.net/manual/en/  is one type of index (a table of
contents).

http://www.php.net/quickref.php  is another type of index (a listing of
all documented functions).

Thank you for your answer.
The table of content, although very useful (we could not do without it),
is not an index.
I checked the quickref.  It is already something, but kind of limited. 
The complete index would allow you to find any important word that
exists in the manual.
For example, I searched the word "if" to see if I would get the if
statement's syntax, an explanation, and an example of an if statement;
although I chose to search through the whole site, nothing was found...
The only place where I could find an example of "if", was in
Functions/User-defined function, where there is an example of
conditionnal function, without showing specifically the "if", and
without discussing the different possibilities of embedded ifs, etc...
Maybe it exists somewhere, but I could'nt find it.  This is where a
complete index would be very helpful.
Thank you.

Not only that, there is a search function in the manual.  What more do
you want?

Search is enough.