Description:
------------
According to the JSON specification, a number cannot start with a 0 nor with a +.

See section 6:
"Leading zeros are not allowed."
"number = [ minus ] int [ frac ] [ exp ]"
http://www.faqs.org/rfcs/rfc7159.html

Therefore, any string starting with a 0 or a + should fail the JSON_NUMERIC_CHECK and not be cast to an int.

Test script:
---------------
print_r(json_encode(array("shouldBeString0123456" => "0123456", "shouldBeString+0123456" => "+0123456", "isInt" => "123456"), JSON_NUMERIC_CHECK));

Expected result:
----------------
{
    "shouldBeString0123456":"0123456",
    "shouldBeString+0123456":"+0123456",
    "isInt":123456
}

Actual result:
--------------
{
    "shouldBeString0123456":123456,    // Bad
    "shouldBeString+0123456":123456,   // Bad
    "isInt":123456                     // Good
}