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<?php $s = array ( '$and' => array ( 0 => array ( 'external_identity' => array ( '$elemMatch' => array ( 'type' => 'bpo', 'value' => array ( '$in' => array ( 0 => '34', 1 => '17753', 2 => '7828', 3 => '35060', 4 => '21', ), ), ), ), ), ), ); var_dump(json_encode($s));
based on iec15
Output for 5.6.38 - 7.3.0rc3
string(113) "{"$and":[{"external_identity":{"$elemMatch":{"type":"bpo","value":{"$in":["34","17753","7828","35060","21"]}}}}]}"