1.) use $camelCase for variables instead of underscores.
2.) primitive types in PHP should be lowercased. Use (string) instead of (String). Also consider using the function strval().
3.) protected member variables should not be prefixed with underscores.
4.) indentation is broken. Make sure you're using the correct number of spaces.
5.) @param documentation should not have --. The documentation generator will format it properly.
6.) give it a run through phpcs. It will pick up a lot of things like indentation and whitespace rules.
7.) consider a different license than the PHP license. The PHP license is great for PHP, but not great for PHP libraries.
8.) The package could use more documentation for what it actually does. After reading the package description I know that it does encryption/decryption using the SHA hash chain. I have no idea why I'd want to use it to encrypt things vs other methods, and I have no idea how the algorithm works and no idea what HCE stands for.
9.) you say you've tested for PERL compatibility. Perhaps you could include those tests with the package.