Description:
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I know that the code below is senseless in this form, but I came across it accidentally.

This code prodcues a dump of $b object variable which contains two members - 'd' and a strange member shown as ':private'. The name is ':private' when the last line is not present. When this line (echo $a->{4} or similar) is put in the script, var_dump ($b) starts to produce a different dump on each script run. A sample output is shown below.

Reproduce code:
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<?
$a['d'] = 4;
$a[''] = 5;
$b =(object)$a;
var_dump($b);
echo $a->{4};


Expected result:
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Maybe

object(stdClass)#1 (2) { array (['d'] => int(4), [''] => int(5)) }

Actual result:
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A sample result is

object(stdClass)#1 (2) { ["d"]=>  int(4) ["?:private"]=>  int(5) }

But '?' is often another symbol or a sequence of symbols. Also it is sometimes just [":private"].