How a PHP interpreter works

Understanding PHP Code Execution Process: A Detailed Insight

PHP, like many other languages used for web applications, is considered an interpreted language. When we execute a PHP application, we often overlook the intricate process that occurs behind the scenes. This article delves into the inner workings of a PHP interpreter, shedding light on how it processes your code.

Compilation vs. Interpretation: Unraveling the Difference

In the realm of programming languages, a crucial distinction exists between compiled languages (e.g., C, C++) and interpreted languages (e.g., PHP, Python, Ruby). Compiled languages undergo a one-time transformation into machine code, eliminating the need for recompilation. In contrast, interpreted languages employ a separate application, the interpreter, to translate code in real-time. This approach sacrifices some performance but offers unparalleled flexibility and ease of development. This section dissects the PHP interpreter’s operation.

The Vital Role of Zend Engine

The PHP language relies on the Zend Engine, serving as both its core and execution mechanism. Comprising a source code to bytecode compiler and a virtual machine, it manages the entire code processing journey. From the moment your HTTP server initiates the execution of a PHP script to the generation of HTML code, Zend Engine orchestrates it all. The PHP script’s processing unfolds in four stages:

1. Lexical Analysis (Lexing): This initial phase transforms the source code into a sequence of tokens. These tokens provide a description of each encountered value and assist the interpreter in further code processing. PHP utilizes the re2c lexer generator, which employs regular expressions to identify code elements like “if,” “switch,” or “function.”
2. Syntax Analysis (Parsing): Parsing follows lexing and involves the conversion of generated tokens into an organized data structure. PHP employs GNU Bison based on a BNF file containing the language’s grammar. This process generates an abstract syntax tree (AST) that serves as the foundation for the compilation phase.
3. Compilation: PHP, without Just-In-Time (JIT) compilation, compiles the AST into OPCode, not machine code. Recursively traversing the AST, this phase may involve optimizations such as arithmetic calculations or the replacement of expressions like strlen("test") with direct values like int(4).
4. Execution: The final phase executes the generated OPCode on the Zend virtual machine (Zend Engine VM). The output mirrors the expected result of the script, often HTML code for web applications.

Optimizing with OPcache and JIT Compiler

The introduction of OPcache has streamlined the PHP interpretation process, effectively skipping multiple steps until the execution phase. Moreover, PHP 8 introduced the JIT compiler, enabling direct execution of machine code, bypassing interpretation or execution by the virtual machine. Previously, there was an option for code transpilation, such as HipHop for PHP, but it was eventually replaced by the HHVM project based on JIT compilation.

A Closer Look at Interpretation Steps

Let’s explore the individual interpretation steps in more detail:

1. Lexical Analysis (Lexing)

Lexing, also known as tokenizing, converts PHP source code into tokens. These tokens represent the meaning of each value encountered in the code. While the actual lexer is more complex, you can get an idea of its function with a simplified example:

Additionally, you can inspect the generated tokens for a sample code snippet:

The generated tokens for this code snippet include elements like T_OPEN_TAG, T_VARIABLE, and T_LNUMBER, along with characters like =, ;, and ? considered as tokens themselves.

2. Syntax Analysis (Parsing)

Parsing involves processing the generated tokens into a structured data format. PHP employs GNU Bison to convert the language’s context-free grammar into a more useful, cause-and-effect grammar. The LALR(1) method ensures that tokens adhere to grammar rules defined in the BNF file. This phase results in the creation of an abstract syntax tree (AST), which serves as the basis for compilation.

3. Compilation

PHP, without JIT, compiles the AST into OPCode. This compilation process includes various optimizations, such as arithmetic calculations and constant folding. Tools like VLD or OPCache can provide insights into the generated OPCode’s structure.

4. Execution

In the final phase, the OPCode is executed on the Zend virtual machine. This execution produces the desired output, often in the form of HTML code for web applications.

In Conclusion: Unveiling PHP Code Processing

Understanding the intricate process by which PHP code is analyzed and executed can greatly benefit developers. It provides insights into security and performance aspects of PHP projects. While most users may not delve into the inner workings of PHP, this knowledge is invaluable for those responsible for server and application monitoring.

This comprehensive overview has delved into the stages of PHP code execution, from lexing to compilation and execution, offering a deeper understanding of the interpreter’s role in web development.