The operators page at /manual/en/language.operators.php lists the the assignment operators (=, +=, etc.) as being left associative, but they must logically be right associative. Left associativity would mean that this:

    $a = $b = 0;

was interpreted as:

    ($a = $b) =0;

which is clearly nonsensical.  Indeed a quick test shows that:

    $d = $c += $a += $b += 4;

is interpreted as:

    $d = ($c += ($a += ($b += 4)));

which looks like right associativity to me!

(Actually, a little further testing suggests that the assignment operators need some special annotations, as they don't appear to strictly obey their stated precedence level.  For example, the statement:

    $x = $z + $y = 543;

if interpreted strictly according to the precedence rules should be equivalent to:

    $x = ($z + $y) = 543;

which would be a parse error, but in fact is interpreted as:

    $x = $z + ($y = 543);

which may be the "obvious" intention, but had me WTF-ing for a while!!)