The three SDO components - the SDO core, the XML DAS and the Relational DAS - are packaged together with Service Component Architecture (SCA) into one PECL project, SCA_SDO, so you can download SCA and all three parts of SDO with the command:
This command will build the SDO shared library as well as installing the PHP files that make up SCA and the SDO Relational DAS.
pecl install SCA_SDO-beta