Decompilation of the binary code of the PHP virtual machine (Zend Engine) involves the process of transforming bytecode back into PHP source code or a form close to it. This is a complex task that requires a deep understanding of the bytecode structure and the workings of the PHP virtual machine.

Main Steps of Decompilation

1. Reading the Bytecode: The first step is reading the bytecode generated by the Zend Engine. This bytecode is an intermediate representation of the original PHP code and is executed by the virtual machine.

2. Analyzing the Bytecode Structure: It is necessary to understand the structure and format of the bytecode. The bytecode consists of opcodes (operation codes), which are commands for the virtual machine, and operands, which are the arguments for these commands.

3. Converting Opcodes to High-Level Constructs: Each opcode corresponds to a specific operation in the original PHP code. The decompiler must convert these opcodes back into the corresponding PHP constructs, such as variables, functions, loops, and conditions.

4. Reconstructing the Source Code: After converting the opcodes, they need to be assembled into a structure that closely resembles the original code. This includes restoring variable names, functions, and other identifiers, as well as structuring the code according to the program's logic.

Tools for Decompilation

There are various tools and libraries for analyzing and decompiling PHP bytecode:

• VLD (Vulcan Logic Disassembler): One of the most popular tools for disassembling PHP bytecode. It provides a detailed output of the bytecode structure and its opcodes.
• PHP Decompiler: Specific tools for decompiling PHP code, which can transform bytecode back into PHP source code.

Examples of Using VLD

VLD allows you to get a textual output of the bytecode, which is the first step in its analysis and decompilation. Example of usage:

bash
php -d vld.active=1 -d vld.execute=0 script.php

This command example enables VLD to analyze script.php and outputs the result in a textual format, showing all the opcodes and their operands.

Challenges and Limitations

• Loss of Information: Some aspects of the original code may be lost during the compilation into bytecode. For example, comments, specific naming conventions, and code formatting.
• Obfuscation: If the original code was obfuscated before compilation, decompilation becomes significantly more challenging.
• Optimizations: Some optimizations applied by the compiler can make it difficult to convert bytecode back into understandable and maintainable source code.

Conclusion

Decompilation of the binary code of the PHP virtual machine is a complex and multi-step process that requires a deep understanding of the internal workings of the Zend Engine and the bytecode structure. Although fully restoring the original PHP source code from bytecode can be difficult or even impossible, using tools like VLD can provide valuable insights into the operation and structure of PHP programs.