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<?php preg_match('/b(?P<a>a)?/', 'xxxxbaxxxx', $matches); var_dump($matches); var_dump(array_key_exists('a', $matches));
Output for 4.3.3 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(3) { [0]=> string(2) "ba" ["a"]=> string(1) "a" [1]=> string(1) "a" } bool(true)
Output for 4.3.0 - 4.3.2
array(2) { [0]=> string(2) "ba" [1]=> string(1) "a" } bool(false)