Having same problem. The only PHP version that has working JSON and PDO is 5.4 native. All the configurable options (5.4, 5.5, 5.6, 7.0, 7.1) fail to find JSON and PDO functionality.
Doing a phpinfo() shows on 5.4 native
json and pdo enabled as shared with the following under the configuration section.
PDO PDO support enabled
|PDO drivers||mysql, sqlite|
pdo_mysql PDO Driver for MySQL enabled
|Client API version||mysqlnd 5.0.10 - 20111026 - $Id: c85105d7c6f7d70d609bb4c000257868a40840ab $|
Directive Local Value Master Value
pdo_sqlite PDO Driver for SQLite 3.x enabled
But on 5.4, 5.5, 5.6, 7.0, and 7.1 (where modules are configureable) the "Configure Commands" are shown for json, pdo, with-pdo-sqlite, with-pdo-pgsql, with-pdo-dblib, with-pdo-firebird, and with-pdo-odbc. However, they don't ever seem to be loaded under the configuration section. Aside from that I have pdo_mysql checked in the module options for each PHP version, but it never showed up in the "Configure Commands" or in the configuration section.