extension_dir: /usr/lib/php4
Try ini_set: FAIL
After ini_set: /usr/lib/php4

Try dl with just filename:
dl cmd.so: FAIL
error: dl() [<a href='function.dl'>function.dl</a>]: Unable to load dynamic library '/usr/lib/php4/cmd.so' - /usr/lib/php4/cmd.so: cannot open shared object file: No such file or directory