I think ',' is used to divide expressions just like C/C++, isn't? Therefore, 

echo 1,2,3,4; 

as well as

$i=0;
echo ++$i, ++$i, ++$i;

works.

http://en.cppreference.com/w/cpp/language/operator_precedence
http://www.difranco.net/compsci/C_Operator_Precedence_Table.htm

I'm not sure what do you mean by "the operator precedence table seems to be in error."

> I think ',' is used to divide expressions just like C/C++,
> isn't?

It doesn't seem to be in the general case.  For instance:

  $a = (1, 2); // => syntax error, unexpected ','
  
Furthermore in C the comma operator doesn't simply divide
expressions, but is defined to evaluate the first operand
discarding the result, and than to evaluate the second operand
"returning" its result.
  
> echo 1,2,3,4;

This is possible due to the special grammar rule echo_expr_list.

> I'm not sure what do you mean by "the operator precedence table
> seems to be in error."

The *only* case where the comma can be used as *operator* seems to
be inside for_expr. However, the documentation states the comma
operator has *many* uses.

Reasonable discussion.

Since this is documented for a long time, it may be good to categorize as engine issue.

> Since this is documented for a long time, it may be good to
> categorize as engine issue.

Please note, that I do not suggest to implement a universal comma
operator as in C/C++. Actually, I believe that it is not necessary
and doesn't have real benefits (at least in PHP).

IMHO it would be sufficient to document the comma operator more
clearly, or perhaps, to not categorize it as *operator* at all
(Java does not have a comma operator[1], but similar uses of the
comma as in PHP).

[1]
<http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.27-120>

@yasuo: This is docs problem. PHP has no comma operator, it only uses commas as an expression delimiter in some places, like function arguments.