Description:
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It is not possible to assign a numeric (integer or string) property name directly but via a variable.

Test script:
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<?php

$obj = new stdClass;
$zero_int = 0;
$zero_str = "0";

$obj->$zero_int = "bar"; // works
$obj->$zero_str = "bar"; // works
$obj->0 = "bar"; // Parse error: syntax error, unexpected T_LNUMBER, expecting T_STRING or T_VARIABLE or '{' or '$' in foo.php on line 9
$obj->"0" = "bar"; // Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING, expecting T_STRING or T_VARIABLE or '{' or '$' in foo.php on line 10

print_r($obj->$zero_int); // works
print_r($obj->$zero_str); // works
print_r($obj->0); // Parse error: syntax error, unexpected T_LNUMBER, expecting T_STRING or T_VARIABLE or '{' or '$' in foo.php on line 13
print_r($obj->"0"); // Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING, expecting T_STRING or T_VARIABLE or '{' or '$' in foo.php on line 15

?>

Expected result:
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I expected that both ways or neither of them would work.