Understanding PHP binary decompile.

Decoder (decompiler) is a software written in various programming languages (C, C#, PHP, etc.) that implements some engineering methods.
The PHP programming language has been developing rapidly for decades and is available to everyone. The source code is interpreted in the Zend Engine environment, using various extensions, runtime extensions.

Decompiler in its current version must solve, implement various mechanisms for decoding and working with the binary code of the PHP virtual machine. You can read older RAID research in "Security Analysis of PHP Bytecode Protection Mechanisms" by Dario Weiber, Johannes Dahse and Thorsten Holz or other research all over the web.

The PHP virtual machine uses a set of operation codes (opcodes, the result of internal compilation) that can be translated back into source code if desired.
VLD, Xcache, OPcache, and the Nikic PHP Parser are auxiliary in the study and can be the basis of any targeted loopback project. Decoding involves working with opcodes and logic. Each PHP version has ~200+ opcodes, and the bytecode structure changes with each iteration of PHP. New functionality is added, and some becomes irrelevant. "Decryption" is a misnomer for the decompilation process because it does not return a reference value. Decompilation is the correct term.