I've developped a php script and I want to protect my files, so what do you think is the best choice for doing this.
Is there any other cheaper tools or even free ?
No amount of obfuscation will protect your files. If they are sufficiently popular, someone will decode them and distribute the versions that don't need IonCube or Zend to decode.
DRM won't stop piracy, but it will get in the way of your paying customers. You'll limit your potential customer base to those that can run the loaders to decrypt your software, which is a much smaller pool of people than all people with PHP web hosting. You'll also vastly increase your customer support load as you help people install the loaders and troubleshoot why your software won't run.
Most importantly, by creating this extra work and frustration for customers, and by preventing them from customizing their copies of the script, you'll lower their happiness with your product. That will result in less referrals, less positive reviews on blogs and social media, and in the end, less sales for you.
The best thing you can do to protect your files AND your sales is to not use DRM. Protect your business by offering incentives to be a legitimate customer, like technical support, free minor version upgrades, customer only discussion forums, etc. Not only will these make it desirable to purchase the script from you instead of download it from a pirate without those benefits, but it'll make your customers happier and more likely to spread the word, leading to more customers.