The Subversion functions work quite well for me, after some searching. I needed some time though to find out how they all worked together, but this is a basic example of svn_fs_is_file:
<?php
$repos_handle = svn_repos_open('/var/lib/svn');
$fs_handle = svn_repos_fs($repos_handle);
$youngest_rev = svn_fs_youngest_rev($fs_handle);
$fs_rev_handle = svn_fs_revision_root($fs_handle, $youngest_rev);
print_r(svn_fs_is_file($fs_rev_handle, '/a-file.txt'));
?>
There is one important thing to note about this all. You cannot let the handles expire while doing any calls to svn_fs_*. When implementing a helper class, I cached the first and third handle, but not the second one. PHP crashes hard when you do this. Keep references to all handles you get while you're calling the svn_fs_* methods.
