Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This syntax is simply not supported, and that is expected.

Actually documentation has nothing about comma expression, the only way to know about it is to view example 4 of 'for' operator. Though, it is also mentioned but not explained in Operator Precedence part of chapter 15.

Well, this syntax works inside 'for' structure, so it is supported, why not to allow to use this syntax in other expressions also?

Because it doesn't make any sense elsewhere.
(Disclaimer: I can not think of any other places where it would be useful, your example is not the best one..)

How about following examples:

while ($a = mysql_fetch_array(),$rec_no++) {
}

$to_pay = ($left<$total)?($left,$stop=1):$total;

I find that very confusing and stupid syntax. Just put the stuff inside the loop like in all other languages. Don't reopen this anymore, this is no bug.

Other languages? Let's see:

[dmitry@srv dmitry]$ cat x.c; gcc x.c; ./a.out
#include<stdio.h>
int a,b,c;
main()
{
        c = a=1,b=2;
        printf("a=%d b=%d c=%d\n",a,b,c);
}
a=1 b=2 c=1

[dmitry@srv dmitry]$ cat x.pl;./x.pl
#!/usr/bin/perl
$c = $a=1, $b=2;

print "a=$a b=$b c=$c\n";
a=1 b=2 c=1

Guess the result of following:
<script>
c = a=1,b=2;
alert('a='+a+' b='+b+' c='+c);
</script>