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<?php $buffer = '{ "format": { "rule": [ "matches", "" ], "message": "Applications.submit_birthdate_format" } }'; var_dump(json_decode($buffer), json_last_error(), JSON_ERROR_SYNTAX);
Output for 7.0.0 - 7.0.20, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
object(stdClass)#2 (1) { ["format"]=> object(stdClass)#1 (2) { ["rule"]=> array(2) { [0]=> string(7) "matches" [1]=> string(0) "" } ["message"]=> string(36) "Applications.submit_birthdate_format" } } int(0) int(4)
Output for 5.5.24 - 5.5.35, 5.6.8 - 5.6.28
object(stdClass)#1 (1) { ["format"]=> object(stdClass)#2 (2) { ["rule"]=> array(2) { [0]=> string(7) "matches" [1]=> string(0) "" } ["message"]=> string(36) "Applications.submit_birthdate_format" } } int(0) int(4)

preferences:
179.06 ms | 402 KiB | 213 Q