Zend Optimizer requires the appropriate files (see the installation instructions) to be uploaded in forced binary mode. This is usually accomplished using a FTP program.
If not uploaded in forced binary mode, you may see one of the two errors below (among others):
Fatal error: Unable to read X bytes in {your php script} on line 0
Incompatible file format: The encoded file has format ID 0, which is too old
Make sure that you transfer files as binary and not ASCII when using FTP to transfer encoded files between computers.
Also, in order to run encoded files you need to install Zend Optimizer. This error may indicate that you have an old version of Zend Optimizer installed.
In order to download the latest version of Optimizer, go to: http://www.zend.com/products/zend_optimizer