And more example：
<?php
$a = 1,$b = 1,$c=1;
$x = $a + 0 + $a++; //$x = 2;
$y = 0 + $b + %b++; //$y = 3;
//but
$z = $c + $c++; //$c = 3
?>


?>

Mixing ++ and + in the same statement provides undefined behaviour in many languages, including C— depending on optimisation etc. In PHP, this also provides undefined behaviour. I'm marking this as a doc issue, as I can't find anything on http://uk.php.net/manual/en/language.operators.increment.php about this.

replay : derick@php.net

example：
<?php

$a=1;
$c=1;
$a = $a + $a++ + ++$a;     //now $a=6; Why? It is right? How it is interpreted?
$c = $c + 0 + $c++ + ++$c; //now $c=5; Why difference between $a and $c?

//In C language or JavaScript a=5;c=5;
?>

Like Derick said, this is undefined behaviour.  Not just in PHP but in most 
languages.  Read this about similar undefined behaviours in C, for example:

http://c-faq.com/expr/seqpoints.html

Asking for an explanation of undefined behaviour is non-sensical.

Thanks for answers!

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

I've added an example on the Operator Precedence page.