PDO is always installed for php 5.1+. You can check for specific db drivers that are installed or not using phpinfo(); You could try to check for specific drivers using @Mark Baker idea and checking for specific constants;
var_dump(defined(PDO::MYSQL_ATTR_LOCAL_INFILE)); // mysql
var_dump(PDO::FB_ATTR_TIME_FORMAT)); // firebird
Note that not all drivers have specific constants defined so phpinfo() remains best solution.
Using command line you can check using:
$ php -m
As an alternative of phpinfo() you can use:
extension_loaded ('PDO' ); // returns boolean
// or
extension_loaded('pdo_mysql');
// or get all extensions and search for a specific one
get_loaded_extensions();
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