PHPC-1491: Replace string read preferences with constants
PHPC-1491: Replace string read preferences with constants

jmikola/mongo-php-driver,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype