To use the ZXID PHP extension you must add near beginning of your script
dl("php_zxid.so"); // Load the module
You may need to tweak the paths, or LD_LIBRARY_PATH, to get this to work.
After this, you can use the PHP interface much the same way as you would use the C interface. See the distributed zxid.php and zxidhlo.php for further usage examples.