Description:
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When a ReflectionType of a nullable type is obtained, its string representation does not include the question mark (?) prefix. This behavior seems to have changed in 7.1.0RC3.

See https://3v4l.org/c21Nq

Test script:
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<?php

$r = new ReflectionFunction(function (?int $a): ?int {});
var_dump((string) $r->getParameters()[0]->getType());
var_dump((string) $r->getReturnType());

Expected result:
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string(4) "?int"
string(4) "?int"

Actual result:
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string(3) "int"
string(3) "int"