Description:
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assign by reference in conjunction with arithmetic does not result in a syntax error as expected. The test script shows two examples. The first one is correct according to the current implementation, but should result in a parser error IMO. The second results in a parser error (how it should be). 

It seems like the parser has a wrong precedence. I think the problem is this line in the grammar:
http://lxr.php.net/xref/PHP_TRUNK/Zend/zend_language_parser.y#777

The langspec does not mention operator precedence explicitly (which should be added IMO) but implicitly the =& operator can be found in the section "Assignment Operator" (https://github.com/php/php-langspec/blob/b1e7a65fb9c985a8114322330468b77fb955cfae/spec/19-grammar.md#assignment-operators)
which is further below than +

Test script:
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$a = &$a + 1;
$a = 1 + &$a;

Expected result:
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parser error in both cases or the precedence explanation needs to be changed. 
Reading the spec I would assume the first line in the code above is equivalent to:
$a = (&$a + 1);
and not
($a = &$a) + 1;