Here is a function that returns an array with the directories in the root of the HEAD revision of a repository, using only the path of the repository.
<?php
    function get_repo_dirs($path){
        if (file_exists($path.'/format'))
        if ($repo = svn_repos_open($path))
        if ($repo_fs = svn_repos_fs($repo))
        if ($head = svn_fs_youngest_rev($repo_fs))
        if ($repo_fs_root = svn_fs_revision_root($repo_fs,$head))
        return array_keys(svn_fs_dir_entries($repo_fs_root,'.'));
        else return false;
    }
?>

example usage

<?php
    var_dump (get_repo_dirs('/home/user/svnrepos/example_project'));
?>