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<?php $arr = (array)json_decode('{"9999":true}'); var_dump($arr, key($arr), isset($arr['9999']));
based on fB0G6
Output for 7.2.0
array(1) { [9999]=> bool(true) } int(9999) bool(true)
Output for 5.3.17 - 7.1.10
array(1) { ["9999"]=> bool(true) } string(4) "9999" bool(false)