Is there any tool or way to decode PHP code?
Further more, how do we know you are the true owner? And are entitled to the source code?
What do you mean “decode” PHP code? “decode” to what?
Nope
A script is encrypted by zend or … . Is there any way to decrypt it with out knowing the encrypted methodology?
I sure have.
And every decrypting method I’ve tested out has been completely useless.
Unfortunately you are in a dead-end situation. Without the original source-code, you have no choice but to write it again.
This is one of the key problems with such encrypting methods. Companies often cause themselves huge problems with them, sometimes causing more financial pain from having to create a fresh write than financial gain from people not being able to copy the sourcecode.
Have you heard about ion cube?
No.
However, and this is mere personal opinion, a PHP script built 6 years ago would now be so out of date that it wouldn’t be worth keeping in the first place.
It is our own script. The encrypted log file is missing. It was build 6 years back. And there is no back up. Thats the issue.
I know its like encrypting database using md5 which is almost impossible to retrieve. Still i would like to know is there any possible way.
It can actually be faster to write your own version of the code, believe it or not 
This is one of the key features with such encrypting methods.
If the code was encoded using one of the major encoders: In the PHP file, there is usually an if-else statement. The first portion usually says something to the effect of…if module x is not installed, here’s a link where you can download and install it on your server.
If it’s an in-house thing, all bets are off.