Description:
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When trying to serialize a variable by hand you need a way to find out, if this variable contains references between two fields or circular references (recursion). Currently there's no way to do this without doing crappy tests (modifying the variables in questions and check for changes being reflected in the other one).

Reproduce code:
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About 50 LOC (See my comment on Spotting References at http://php.net/manual/en/language.references.spot.php)

Expected result:
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//Using internal functionality ;-)
$a = is_ref($var1, $var2);

Actual result:
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Slow reference testing due to much overhead by not being supported by the PHP internals ... internally this would be about 10 LOC IMHO.